In a discreet corner of the Caramel Room at the Berkeley Hotel in Knightsbridge, Pierre Koffmann, perhaps the greatest chef London has ever known, hands me three pieces of A4 paper. "Here it is," he says mildly, as if I were his accountant, and the documents something to do with his VAT return. "The menu."

There is no drum roll, no blast of heavenly trumpets. Angels do not sing. To my right, a thin woman in pearls and expensive shades of beige is laughing at something her companion has said, all the while pushing salad leaves from one side of her plate to another. Beyond her, a group of businessmen are talking a strange international language that seems to consist mostly of acronyms and the words "for sure". All is perfectly ordinary, at least in Knightsbridge terms; the affluent life of the hotel goes smoothly on. No one appears to be aware that, over on my side of the room, history is being made.

I look at the menu. It is exquisite: simple, delicious, la France profonde distilled and neatly typed up. There are 14 starters, among them vinaigrette of leeks with smoked eel, potted foie gras with grilled bread, and Bourguignonne snails with wild mushrooms and bone marrow, and 17 main courses, from which I would obviously choose either Koffmann's signature pig's trotters with morels and veal sweetbreads or the daube de joue de boeuf (though if you are a less greedy, less meaty kind of a person than me, you could, I suppose, order weedy old loin of tuna Provençale, or Dover sole). Then there are the puddings. Oh my God. This is good. Koffmann's famous pistachio souffle is here, of course, but there is also croustade aux pommes caramelisées, a strawberry vacherin and a mousse au chocolat. These are the puddings I ate as a little girl when I visited France for the very first time, and all I knew was Ski yoghurt and lemon meringue pie.

Nevertheless, there is, I think, something missing and the words are out of my mouth before I can stop them. "I can't believe you haven't put îles flottantes on," I say, peevishly. "I am campaigning for the return of îles flottantes in smart restaurants everywhere, and I was relying you to get the whole thing started." Koffmann looks up. He has been reading the menu, too. He tells me that the dish I have in mind – yielding clouds of poached sugary egg white floating on a lake of smooth custard and decorated with a sparkling web of crisp caramel – is properly known as oeufs à la neige (snowy eggs as opposed to floating islands). Okay, then. Oeufs à la neige. Whatever. Put them back on the menu. After all, they are a confection so light, he could probably pass them off as a diet food to the ladies who lunch.

The corners of Koffmann's mouth turn down. He doesn't actually say the word "no", but he doesn't have to: his entire being exudes it. He has written his menu and that, for the moment, is that.

Recession or no recession, big-name chefs seem to open new restaurants in the capital almost every week. In the case of Pierre Koffmann, however, there is no need for a PR campaign; the anticipation, based as it is on memory and experience – during the years he was chef patron at La Tante Claire in Chelsea, Koffmann's cooking was considered to be the best in Britain by a country mile – is real. By the time you read this, the doors of his eponymous new bistro on the site of what was previously Gordon Ramsay's Boxwood cafe at the Berkeley will still not quite be open, yet I would be willing to bet that tables will already have been booked some way into next month, probably beyond. This is the return of a true master. "London should be very happy indeed to have him back," says Tom Kitchin, one of Koffmann's many protegés (Kitchin's restaurant in Leith, Edinburgh, was awarded its first Michelin star six months after it opened in 2007). "The skill of the man is incredible. He can extract flavour from anything."

The rumours had, of course, long been swirling, like steam over a stock pot. Koffmann shut up shop in 2003 (by this time, he had moved La Tante Claire to the Berkeley), and went fishing. I mean this literally. "Yes, I went fishing," he says. "And I travelled, and I played golf, and I went mushroom picking." Quite quickly, though, he started to feel listless. "I was getting bored. I was staying in bed until nine in the morning. I'd just about manage to go out and get a coffee. Then, at 11, I would feel hungry, so I would go for lunch. Then I'd come home for a siesta. I thought: I'm too young for this [he is 62 next month]. You cannot go fishing every day! You cannot go mushroom picking every day! If you play golf every day, it's not a pleasure, it's just another job."

He started doing a little consulting work, just to keep his hand in. Then he did a stint as head chef at the Bleeding Heart in Clerkenwell. Finally, last year, he agreed to spend two months running a temporary "pop-up" restaurant at Selfridges. Was it a success? You could say that. A girl could more easily have landed a starring role in a Hollywood movie than she could have bagged herself a table at this joint. I know; I tried.

"I've got to be honest," he says. "The first month [at Selfridges] was a kind of hell. It was so hard: 12 hours a day, seven days a week. But the second month was easier. I got used to the place. I started to think about a new restaurant. I thought: why not? I still enjoy it. When you are a chef, your place is in the kitchen. I'm not a chef outside. I'm not even a chef at home: Claire [his partner] is the chef. I have to do what she wants." He grins. "You know, when you think about it, you should be bored in the kitchen. After all, you have to do the same things every day. You arrive in the morning, you check the meat, you check the fish. But every day is different. Sometimes, the sea bass is not so beautiful, and sometimes it is so beautiful, you want to kiss it. So it's a very exciting chore." He realised that he had missed the buzz: the noise, the smells, the sudden changes of mood: "Sometimes, you shout. Sometimes, you laugh." So when the Berkeley offered him a berth, he said yes pretty quickly. "We had lots of interest, lots of offers. But theirs was the one that suited me the best. I am too old to be the patron."

This time around, he is not going for Michelin stars (for a long while, he had three). He will now cook "from the heart". The menu is a sophisticated take on the Gascon food he knew as boy. There will be stews and confits and cassoulets; there will be black pudding and rabbit and tranche de tête de veau pané gribiche. He says now that stars were never his thing; that the acclaimed dishes he turned out at La Tante Claire were, towards the end, perhaps just a little fussy for his taste. Did the pressure ever get to him? He shrugs. "There are chefs with Michelin stars who kill themselves, and there are people who work in the post office who kill themselves. Who knows why they do it? I didn't really find it a pressure. When I moved here [to the Berkeley], I lost one star. That's life. You get them, you lose them; it's not a big deal. When I got my third star, I should have been jumping as high as the ceiling. But I wasn't. What is the best restaurant in the world? It doesn't exist. There are some restaurants with stars that are empty, and others without any that are full. Your local restaurant can be the best because this isn't only about food. When I first came to England 30 years ago, the guides used to put food third, after service and ambience – and, in a certain kind of way, they were right. It's a bit pretentious to say that only the food matters." Before you fall off your chair in amazement at this, remember: it's easy to be blasé about such things when you yourself can cook like a god.

I am supposed to be trailing Pierre Koffmann in the supposedly frantic weeks before his new place opens. But this is easier said than done. I can follow him, but there is nothing to see. I had expected panic: frayed tempers, nerves. Instead, I am faced with a cliche of the phlegmatic Frenchman; mostly, when we meet, he is drinking coffee, or working his way through a bowl of salted nuts. The only outward sign that he is about to step up to the (exceedingly hot) plate once again is that – or so I overhear – he appears to have hired a personal trainer. He chooses his new team – his head chef will be Tim Payne, with whom he has worked off and on for 25 years – with the minimum of fuss. Ditto his cutlery and china. And he professes himself pleased with the etching of a little pig that will adorn his plates and menus. About three weeks before opening day, we go down to the dining room, only to find that it is still a mass of concrete and dangling wires. Crikey, I tell him. The builders are going to have to get a move on, aren't they? Koffmann makes a face. It says: indeed. It also says: surely it is time for lunch?

Over the years, there have been suggestions that Koffmann can be a tyrant in the kitchen. Marcus Wareing, who now runs his own restaurant at the other side of this very hotel, once testified that he was a quixotically terrible boss; another anecdote involves Koffmann telling all his staff to clean the floor bang smack in the middle of service. Now I've met him, however, I find this hard to believe. Unless he has simply mellowed. He is rather shy, I suspect, but he is also smiley, and warm, often touching your arm to make a point. What's more, among those who worked in his kitchen – alumni include Tom Aikens and Eric Chavot – I find only adoration. "When I first came to Tante Claire, I was 18 years old," says Tom Kitchin, who worked for him for five years. "Jesus. I was so out of my depth it was really not funny. For the first year or so, I got a good arse-kicking. But then, slowly, slowly, he started to give me a little bit of recognition: a squeeze of the shoulder, a pinch of the thigh. I got stronger and stronger. Something clicked. I began to understand the philosophy of the man; I began to understand how much of an honour it was to work for him. Now, we speak every week. He's like a father to me. The thing you have to understand is that he lives and breathes food. I remember he went to Cyprus once. 'What was it like?' I said. 'Awful,' he said. 'Apart from the chips; in Cyprus they have brilliant potatoes.'"

Helena Puolakka, the executive chef at Skylon, at the Royal Festival Hall, worked with Koffmann for almost five years. "That in itself tells you something," she says. "I have such great admiration for him. He has such authority. He is so creative and inspirational. Whenever guests of his or even just regulars came to the restaurant, he would always give them a special tasting menu. I still get goosebumps thinking about it. He would go to each section of the kitchen gathering the ingredients. It was generous, but it was also very challenging in the middle of service." Are they still in touch? "Yes. He's coming for lunch at my house on Sunday. And I know that I can always call him. He's always there for me."

Koffmann was born in Tarbes, in south-west France, where his father was a mechanic for Citroën. But it is Saint Puy, a village 50 miles north, that we should consider his spiritual home. His maternal grandparents, Camille and Marcel, lived in a Saint Puy farmhouse called the Oratoire and the young Pierre would visit them every school holiday. It was at the Oratoire that he earned the rudimentary elements of cookery and the rhythm of the seasons: trout, wild mushrooms and snails in the spring; melons, apricots and crayfish in the summer; hares, pheasants and ortolan in the autumn; ducks, geese and rabbit in the winter. "The produce was mostly from the farm. Steak was rare; we ate a lot of poultry. My grandmother did own a cooker, but most of her work was done over an open fire. Every Sunday, two chickens. The thing I loved most of all was when my grandfather put a head of garlic to roast in the ashes." In 1990, Koffmann wrote Memories of Gascony, reminiscences and recipes from the Oratoire accompanied by dewy photographs of the soft countryside. In it you will find his grandmother's recipe for walnut leaf aperitif (take 50 walnut leaves, 40g of orange peel, a litre of armagnac, a kilo of sugar and five litres of red wine; macerate for eight days, stirring twice a day; sieve, bottle, and store in a cool place for three months). It is both a sign of the excellence of its recipes, and of Koffmann's popularity, that a first edition of this volume, assuming you are able to track such a thing down, will now set you back up to £275.

In 1963, Koffmann, never exactly a diligent student, left school. "I applied for a few jobs: the railway, the post office, and cookery school. But I wasn't ready to be a man, and that's why I chose cookery school, so I could be a student a while longer. As stupid as that." When he graduated three years later, his report card said: "Pierre will never do anything in catering." But by this time, he had decided that cheffing was a better option than waiting on tables, and so he pressed on in Strasbourg, Toulon, and then – because he wanted to see England play France at Twickenham – a job at Le Gavroche, the Roux brothers' restaurant in London. "It wasn't the most exciting food. It was watercress soup, trout with horseradish, that sort of thing. But the guys were very nice, and when they opened the Waterside Inn at Bray in 1972, they asked if I wanted to run it. I met Annie [his late wife, who was the Waterside Inn's manager], and I stayed for five years, until I opened Tante Claire." Six years later, he won his third star.

I wonder if he thinks British cooking has really changed; if he helped to lead a revolution. Perhaps we are just fooling ourselves things have improved since the 1970s. "In France, you used to be able to take your car, and stop, and you got something nice. The woman was cooking, the guy was in the dining room, the kids and grandmother were helping out. That's completely finished. It's over. In terms of small restaurants, Spain is 10 times better. But in terms of three-star restaurants, the French are still there. In Britain, well, it's good to be proud of what you've achieved but, in general, you're not there yet. Not enough chefs cook from the heart. There's a lack of originality, too much copying. Langoustines and scallops and lentils. It's a bit boring. I prefer a really beautiful shepherd's pie." Where do he and Claire, a potato expert – really! they met over a tray of pink fir apples or something – go to eat? "In the game season, to Le Gavroche. For fantastic tripe, to Arbutus [in Soho]. Sometimes, we just want six oysters and a bit of beef. We go to Le Colombier [in Chelsea] for that. At home, we eat very simply. Claire is a very good cook. She will ask me for advice, but when I give it, she gets cross. The other night we had Bruno Loubet [now at Zetter in Clerkenwell] and Raphael Duntoye [the chef at La Petite Maison in Mayfair] over for dinner. I told Claire to cook. She did pork belly – she's good at crackling – and apple crumble, and they loved it. We all did. We don't want to eat foie gras every night. "

Two weeks before the restaurant is due to open, Koffmann invites me to his kitchen for a cookery lesson. I can choose the main course, but he will decide on the pudding. Wow. As a friend says: imagine how much you'd make if you put a Koffmann masterclass on eBay.

First, we make rabbit with olives. The rabbit, which arrives wrapped in plastic from Allens, the posh Mayfair butcher, looks unpromising. It is French, and farmed. But Koffmann isn't remotely worried. He chops it into pieces and shoves it, head included, into a sauté pan. The meat browned, he adds some stock, wine, several cloves of garlic and a few tomatoes, and shoves it in the oven. When it emerges 20 minutes or so later, he does a bit of fancy footwork with a conical sieve and a wooden spoon, and then he presents to me a stew so delicious-smelling, I know that I simply must eat it, even though it is still only 11 in the morning. Tom Kitchin was right: this man can extract flavour from anything.

But while I nibble at leg and breast girlishly, Koffmann is preoccupied elsewhere. At the other side of our station, he sets about the head with a cleaver. "When I was a boy, this was the bit my father got," he says. "But now.... it's all mine." With a fat index finger, he scoops out the rabbit's brain, then with thumb and index finger he plucks out its cheeks. He eats both with a blissed-out intensity you rarely see in British chefs. It's almost as though he cooked this dish for himself.

Next, pudding. "Oeufs à la neige," he says. "We are going to make oeufs à la neige." And this is what we do. A bowl of custard made from golden egg yolks and vanilla; three fluffy clouds made from whites and sugar (when you come to poach them, squeeze them with your fingers to tell when they are ready); golden brown caramel made from more sugar and just the tiniest bit of water. When at last we put the three together, the plate is a picture. Who could resist such a dish? Not me, that's for sure. And not Pierre Koffmann either. Before I leave, he tells me – relishing the moment, I think – that he has had a rethink. Oeufs à la neige will be making an appearance on Koffmann's menu after all. "Yes, why not?" he says, smiling broadly. "They are very good, no?"

Koffmann's opens on 30 June at the Berkeley hotel, London SW1 7RL. Tel: 020 7235 1010 www.koffmanns.co.uk.