Broadgate, Grasmere, Cumbria (01539 435 217). Meal for two: £70-£100

The small Oak Bank Hotel in Grasmere is not one of the Lake District's fanciest establishments. To be frank it's not even one of the fanciest establishments in Grasmere. At the front there is a comfy lounge with a log fire, and at the back one of those white wipe-down conservatories that you see advertised in Sunday colour supplements. For all the self-consciously modern art on the walls of the bar and dining room, this is very much the sort of place where hill walkers would feel comfortable taking off their boots and warming their stockinged feet by the fire.

It is not the sort of place where you expect to be startled by the quality of the cooking, as in the full "Gosh!" and "Oh my!" and – whispered, so as not to offend the owners – "What is this food doing here?" Stuff of this quality and ambition tends to come with flummery and fuss. Here they just bring you nice plates of food.

Chef Darren Comish arrived quietly back in May and began cooking. He has an interesting CV which includes a stint at the Michelin-starred bit of the Devonshire Arms at Bolton Abbey. It's not that he is terrifyingly original; Lord save us from that. I've seen plates that look like his in other places. It's that almost everything on Comish's plates counts and makes sense. There's brightness and an unusual clarity. And blimey but it's good value. At lunch three courses (four, if you count the exceptional pre-starter) cost £19.95; even in the evening when the choice, along with the largesse, expands, it's still only £35.

The fact that we are the only diners for lunch on a Monday simply adds to the impact. In seclusion you have no choice but to focus on what is placed in front of you. Which is what we do when three mini loaves of warm bread arrive, along with some salted butter and logs of same flavoured with seaweed or lemon and herbs. The lemon is lovely, the citrus and dairy seesawing with each other. You could imagine it melting away on a grilled piece of fish. But oh the seaweed butter! It has a slap of iodine and umami and a deep, creamy end. Suddenly I am perving over butter. We are forced to portion it up.

And then that pre-starter: a lightly battered frog's leg. Beneath is the soothing lick of confited egg yolk. Around it is an intense seafood soup, a bisque by any other name, with a thick dusting of an even more serious seafood "powder" which is the sort of thing you would want to dab at with one fat fingertip. We almost take the glaze off the bowl it's so good and sit staring at the empty porcelain wondering what it had been doing on this table.

A coarse game terrine comes with a crumb of the famed Grasmere gingerbread, which brings the waft of the spice cabinet to the plate. There are splodges of chutney-like purée in among a salad of crisp autumn vegetables and, best of all, tiny cubes of pickled onion jelly. Finely mandolined pickled vegetables turn up as the foil to a tranche of mackerel which has been given a proper spanking on the grill, so the skin is crisp and bubbled. It's what so many mackerel dishes are aiming for but don't quite have the guts to be.

As there are only three starters we conclude it would be churlish not to have them all. So here comes a bowl containing two smoked chicken rissoles, sprinkled with another fighty powder which is the essence of every roast chicken dinner you've ever had. Somebody should flavour crisps with it. Around this is poured a celeriac soup which has all the high, airy, vegetal notes you expect but also something more profoundly savoury. Somewhere in here is a brilliant stock, probably made with the rest of the chicken that provided the powder.

Next, a "tasting of pork". There are tiny onion rings. There are dabs and splodges of butter-rich mash. There is a sherry-based sauce of a richness that manages to stay the right side of something with which you would weatherproof a fence. Most of all there is pig: not just cheek but, as is pointed out, both inner and outer, one denser than the other. There is belly. There is liver, dark and sticky and serious. It is porker central.

A tranche of salted cod has a pleasing meatiness, but eaten alone is just a little too salty. It is the one fault, the one thing we could niggle at. And yet when it comes together with the various riffs on cauliflower and Romesco with which it is served – ground down into a "couscous", caramelised – it seasons the rest of the plate. Even the light "curry" sauce, a notion from the French classical kitchen which all too often traduces the entire culinary tradition of the Indian subcontinent, works here.

There are three desserts, so again we put our backs into it. A sticky toffee pudding is an ethereally light bit of sponge, so not quite the dense puck it should be, but the sticky toffee brûlée and the banana ice cream which crowd on to the plate make up for that. There is an uncommon delicacy and lightness to a mango délice with a layer of frisky gel on top, alongside a "cannelloni" of mousse. Best of all is a single ripe fig, roasted within a thick toffeed pastry shell with a quenelle of cardamom mousse. Most brilliant desserts are a trip to the nursery. They infantilise. This is thoroughly grown-up. You could imagine small children recoiling from the plate; we adults fall quietly in love.

There is a chance to meet the chef, so I take it. I want to know how many people cooked this glorious meal. He shrugs. Just him. It's a Monday and only one booking, so he did it alone. Even on a busy weekend there are only three in there and one of them does the washing-up. I say thank you. I also give thanks for having stumbled across the blog of Cumbria Foodie. I had asked for Lake District recommendations via social media. There were lots of sturdy and hearty-looking pubs. All seemed fine, but hardly worth the paragraphs. Nobody but the Cumbria Foodie had written about Darren Comish and the Oak Bank Hotel. I decided to take a punt. God but I'm glad I did.

Jay's news bites

■ For more food in unexpectedly modest hotel settings, try Allium at the Best Western Abbey Hotel in Bath. Chef Chris Staines made his name at Foliage at London's Mandarin Oriental, before a career diversion which eventually took him to Bath. Try his quail glazed in chilli caramel, or his lamb shoulder pastilla. The £21, three-course set-price menu until 7pm is a steal (abbeyhotelbath.co.uk).

■ It seems playing with your food pays. Double Michelin-starred chef Sat Bains has successfully claimed tax credits for his development kitchen, the ultimate status symbol for any self-respecting gastro pioneer. He argued it was an R&D enterprise of the sort run by hi-tech industries. The tax man cogitated. Then agreed. Sat got the cash (restaurantsatbains.com).

■ The combined award for most shameless over-promising while cashing in on people's vulnerabilities goes to The Fast Metabolism Diet Cookbook: Eat Even More Food and Lose Even More Weight, which is being published on 31  December – just in time for New Year resolutions.

Email Jay at jay.rayner@observer.co.uk. Follow Jay on Twitter @jayrayner1