Barbecoa, 20 New Change Passage, London, EC4M 9AG

Before the TV shows, the bestselling books, the school-food campaigns and the browbeating of obese Americans, Jamie Oliver's approach to cooking was that of an experienced brickie – grab this brick, mix this cement, trowel the cement on here, plonk the mixture down there and bish, bosh, zing, zing, hey presto it's done. He convinced the nation that simplicity, rather than complexity, could deliver big flavours. Through the unveiling of his 15 restaurant, and his immensely popular Jamie's Italian chain, he has kept faith with the basic, the tasty, the honest-to-God. Devotees will be relieved to hear that his newest incarnation mostly maintains the tradition, at least when it comes to food. If only everything else about it were so simple.

Barbecoa is located in the new shopping centre called New Change, beside St Paul's Cathedral. Behind a glass front, you see a big B logo and a picture of a grinning J Oliver. Inside, a woman explains that no, no, this is the Jamie Oliver shop, you silly things. The restaurant is down there, turn right and take the lift to the first floor.

A sweet waitress led us into the startling dining room. It's like walking into a film set – the film being Welcome to Sarajevo. You walk past a brick wall, inset with brass-framed windows, through which you can see an arsenal of fancy barbecue equipment: a Japanese robata grill, a Texan smoker and a Tandoori oven like an anti-personnel device. Through the huge window is a view of St Paul's. Unfortunately, it's the arse-end of St Paul's, behind which are three concrete blocks. "I don't think Christopher Wren designed those," said my friend Lisa smartly. "More like Albert Speer."

The main dining area is designed by Tom Dixon, king of metal light fittings. Here, he has excelled himself with hanging lights in organ-pipe shapes – but they don't go with the banquettes, which are the dirty-rose hue of liver sausage. It's so confusing, you bury your face in the tabloid-sized menu.

The influence of Oliver's partner, Adam Perry Lang, is obvious. Lang is an American "bona fide barbecue expert" whatever that is, who runs Daisy May's BBQ restaurant in New York. He and Jamie are committed to showing off "the depth of flavours that smoke, fire and charcoal can bring to food". So the main courses are few and all are carnal: rump, fillet and strip steak and burger, lamb skewers and leg steak, pork belly, rib and shoulder, spatchcock chicken... Heaven help any vegetarian who walks into this hecatomb of seared meat.

Waiting for the main event, you can marvel at how odd the early dishes are. There's a £4 charge for bread and butter – you get four tranches, of tandoori bread, sourdough, rosemary toast and pumpernickel, but to get your money's worth, you'd have to eat the lot and have no appetite left. My starter of crispy pig cheeks was a classic case of over-production. The pig's head had been roasted until the flesh fell off, the cheeks had been boned, mixed with celery and carrots, rolled up, sliced, then cooked until the top was blackened. Its porkiness was muted rather than enhanced by all this activity, and its indifferent flavour wasn't improved by piccalilli. Lisa's baby back ribs, though, were excellent – the meat came off the bone easily and was properly sticky, with real chilli heat in the marinade.

Then the wait was over and my steak arrived. ("Be careful of that knife," said the waitress. "It's quite sharp." Did I really need to be told that?) It would be rude, in a major new steakhouse, not to order the most spectacular one, so I did – a 400g monstrosity like a slab of roadway, its right-angled bone palaeontologically solid and ancient-looking. It sat on a hillock of limp lettuce with a light jus of garlic, anchovy and lemon, and it was wonderfully, explosively juicy, its texture less clenched than most strip steak, its charred smokiness, a real treat. The chips were also ambrosial, lightly crisped like rectangular planks of perfect roast potato.

Lisa chose the fish of the day, hot salmon on a salad of glass noodles containing pork belly. The salmon was perfectly cooked, neither too dry nor fashionably slithery on the inside – "but it doesn't taste of very much, and I could have cooked it myself at home". As for the presence of pork belly – not so much surf'n'turf as surf'n'sty – they seemed to her "like bits of leftover Pad Thai". A side of creamed spinach was eccentrically topped with breadcrumbs and fried shallots. They gave the spinach "bite" – but I ordered the spinach to complement the steak, not to be given a bowl of spinach-plus-roughage.

We shared a yummy banana split with chocolate curls, butterscotch crackle, vanilla ice-cream, and a rather intrusive walnut and banana-bread base, and split (as we'd say in the Sixties). The war-zone aspect of Barbecoa is hard to love, but when it gets it right (as with the steak) you can forgive the slapdash approach to anything which isn't carnivore-related. Time will tell, but I'm not convinced this is a place to which Oliver's army will flock.

Barbecoa, 20 New Change Passage, London, EC4M 9AG (020-3005 8555)

Food 3 stars
Ambience 2 stars
Service 3 stars

Three courses à la carte £110 for two, including wine

Tipping policy: "Service charge is 12.5 per cent discretionary, of which 100 per cent goes to the staff; all tips go to the staff"