27 High Street, Cardiff (02920 224 817). Meal for two, including wine and service, £80

Opposite the Potted Pig in Cardiff is a branch of High and Mighty, the clothes shop for vast men with their own measurable gravitational field. It was a touching sight in these bland pedestrianised streets at the heart of the Welsh capital, about which nobody will ever write a sonnet, as if it were a marker for largesse in the immediate vicinity. Fill up on piggyness on one side of the road, then roll out across the precinct to find something a little more accommodating to the newly fuller figure on the other. (The safari suit is a classic and I won't hear a word said against it.) I was suddenly attracted to the notion that branches of High and Mighty could behave as the pilot fish of the British restaurant world: if you see one, you know there's somewhere good to eat nearby.

Shame it doesn't actually work like that, but at least in this case it's true, however accidentally. The Potted Pig, which opened earlier this year, is a gift to a city which, even its biggest fans will admit, has rarely been spoilt for good restaurants. For a long time people talked sagely about Le Gallois, but that has now gone. There are a couple of Indians, most notably Mint and Mustard, which did well in the Observer Food Monthly awards – and that's about it.

If I'm wrong on restaurant choices I know you'll tell me, but certainly the Potted Pig, located in reconditioned, brick-lined bank vaults, is a jewel. They describe their food as British, with an occasional nod to French classics in one direction and New York grills in the other, and that just about does it. It is solid, gutsy food without dogma and with quite a lot of pig. They also have around 30 gins, because apparently it goes well with pork, which makes sense on account of the juniper, though I've always found gin a little bullying. Still, there is a smart wine list, which doesn't act as if it's trying to pat you down for cash. Nor does the menu, with starters at £6 and most mains in the low teens.

An "Offaly good breakfast" may be a crap joke, but it was true to its word: grilled kidneys, pink at the eye, with Leopold Bloom's "faint tang of urine", a little fried wobbly liver, a round of black pudding, a fried egg trimmed unto the yolk on a piece of fried bread, a plank of crisp bacon and a dollop of their own ketchup. Cod cheeks and clams came in a buttery persillade that soaked on to the hunk of toast beneath, and made eating it all a two-stage affair. And then, of course, there was the potted pig, which arrived in a Kilner jar, at room temperature, the better to be spread on toast alongside pickles. If you are going to name your restaurant after a single dish you'd better get that dish right. They do.

A whole Devonshire crab with chips and mayonnaise (for £20) was, admirably, just that. A whopper of a whole beast, sprawling off the plate like a hungover student, cracked and cleaned of the less edible bits, and that's it. . A (Welsh) Barnsley chop came with peas, carrots and swede mash and was the sort of thing you'd want in a catacomb on a winter's night.

My only question hung over the Potted Pig cassoulet, which wasn't really. The genuine article is a single combined dish that should take 24 hours to make, into which the crust is constantly refolded. The lumps of pig and duck should be like bits of an ancient wreck on the ocean floor. This was a stew of beans, carrots and sausage on to which had been plonked lumps of pork belly and confit duck. That said, both had skin as crisp as a Welsh winter morning, so I ain't complaining.

After last week's sad attempt at Sticky Toffee Pudding, this week's aced the challenge: light sponge, lots and lots of sauce, their own vanilla ice cream on the side. A lemon posset under a layer ofwith dark berry compôte was just the thing to brighten you up after all that hot pig action. The Potted Pig is not revolutionary. Nor is it trying to be. It simply represents an instinct to feed, using the best British ingredients, and a bit of French technique. The people of Cardiff should be pleased to have it. And that branch of High and Mighty may well get something out of it too.

Email Jay at jay.rayner@observer.co.uk or visit theguardian.com/profile/jayrayner for all his reviews in one place