The Roth Bar & Grill is arguably the coolest gallery café in Britain / Aaron Schuman

Hauser and Wirth Gallery, Bruton, Somerset (01749 8147000)

Very occasionally when you're travelling, it happens. Turning down a darkened side street, drawn by music and the hum of conversation, you find yourself in the perfect bar, filled with cool locals. You pass a dream-like night there, quite content, knowing you're exactly where you ought to be.

What you don't expect is to find such a mirage in a small town in Somerset. Seriously, if this were a video-enabled column, you'd think I'd faked the footage. A dark country lane, the swish of car wheels on gravel. The thump of distant beats, swelling as the camera pans across ancient stone farm buildings to a courtyard filled with outlandish sculpture. Laughter and music spills from a covered terrace packed with interesting people, eating and drinking. Behind them is a wildly decorated bar, conjured out of scavenged planking and blinking video screens. Tracking past an open kitchen, we reach the restaurant, a lime-washed former cowshed, stuffed with a prolific hotchpotch of spectacular, eye-popping modern art.

This is the Roth Bar & Grill, arguably the coolest gallery café in Britain. It's part of the new Hauser and Wirth arts centre, a sensational exhibition space on the edge of Bruton. Given the gallery's roots – the owners are art-scene powerbrokers in London and New York – the place was always going to look remarkable. What I wasn't prepared for was this... scene.

Roth Bar & Grill, named for the artist Dieter Roth, opens for breakfast, lunch and afternoon tea. Only on Friday nights does it explode into a Whisky Galore!-ish fantasia of fun: Blow Up meets Babington House. Young and arty, old and ruddy of complexion and trouser – all Somersorts are here, and they are having a fine old time.

We score the last table in the restaurant, rather than having to wrap ourselves in shawls on the terrace. Conversation stalls as we gawp at the profusion of amazing, baffling art around us. Sculptural chandeliers of tangled neon hang from the roof beams (king post truss, according to my guest, Kevin McCloud). Above our table is a video installation made from plastic garden utensils. Behind us, a slumped chef smokes against a tree, in Rodney Graham's enormous back-lit photo. It's almost overwhelming.

Cleverly, amid this visual bombardment, they've kept the food relatively simple. A few nibbly small plates, some meaty specials using cuts from animals reared on the working farm, a couple of sharing platters, two desserts. It's Soho House-ish in its confident simplicity – to use a reference the clientele would no doubt understand. Wisely, the gallerists have enlisted the services of Catherine Butler to oversee the restaurant; her lovely Bruton local, At The Chapel, is the heart and hub of this community.

Some shared small plates offer low-key pleasure. A heap of devilled whitebait, as yielding and fleshy as sprats, demand to be dunked into a pert aioli. A warm Scotch egg is all crunch and peppery comfort. And a mild-mannered pork terrine comes with griddled sourdough and a punchily acidic chutney.

Things step up a gear with the mains. A sharing platter of Woolley Park poussin is that rare bird – chicken that actually tastes like chicken. Brined and poached then spit-roasted, the spice-rubbed skin basted and burnished to a shine, it's served jointed and heaped on a raised platter, with wedges of skin-on potato dredged with sea-salt and rosemary.

Those same potatoes reappear with a sparklingly fresh tranche of Lyme Bay sea bass, the crisp skin drizzled with gremolata. The fish is one of the few things on the menu not sourced from within five miles. Hyper-local, though, is the turbo-charged watercress which whacks you with iodine wildness. Grown a couple of villages away by organic gardening legend Charles Dowding, it could power a small car.

The Roth's focus on the local and sustainable chimes with the philosophy of the gallery, which positions itself as a community resource, rather than some Frieze-ish interloper beamed down for art tourists. There's certainly a relaxed, convivial feel to the place, which is reflected in the service – surprisingly slick, given that the Roth only opens for dinner once a week.

I don't remember much about the peach Melba and Cambridge burnt cream with which we ended our meal. My notes seem to consist mainly of the words 'buzz' and 'buzzy'. When we leave, a DJ is on the decks, and the place is still jumping. How strange to step from this exuberance into the still, dark Somerset night. If I lived round here, I'd be absolutely thrilled, and not a little smug. A case of 'if you build it, they will come.' And if you also make it really fun, they won't want to go home.

Food ***
Ambience *****
Service ****

Hauser and Wirth Gallery, Bruton, Somerset (01749 8147000). Around £25 a head for three course, before service