The River Cafe's bestselling cookbooks brought polenta to the people. Its menu is legendary among New Labour sophisticates. But what happens behind the scenes? John Walsh finds out

It's 8.45am at the River Cafe. Ruth Rogers, Rose Gray and their head chef, Theo Randall are drinking cappuccinos and working out the day's menu. This daily ritual might seem, to the untrained eye, a happy succession of random gastronomic judgements and desires no sooner imagined than indulged ("We could do the leek and potato and anchovy gratin with the lamb; and maybe peas with the guinea fowl... ") but, of course, it isn't. It's brutally determined by the fall of seasons, the availability of produce from the restaurant's suppliers, and the immaculate taste of the lady proprietresses.

Out the back, modest piles of vegetables, ordered last night, are lying in boxes, waiting to be turned into lunch. Zucchinis, carrots, red potatoes, three types of artichokes, acres of greensward and rain forest that are actually rocket and spinach, crates of slender asparagus, spring onions that turn out to be needle-thin leeks, and long verdant flutes enclosing platoons of broad beans, crouched in their pods like anxious paratroopers waiting for the drop.

They have reason to be nervous. Rose and Ruth, in most other respects two charming, indulgent and tolerant women, represent the toughest quality control department outside a German car manufactory. It's an attention to detail that has made their restaurant a howling success and a temple of sophistication since they set up together 13 years ago.

Rose is a striking-looking woman with a shock of white hair, a curious vulture slouch and the demeanour of a sexy convent headmistress. She is entrancing company, but strict with vegetables. Show her a sub-standard lentil and she'll come on like the Queen of Hearts in Alice, shouting "Off with its head!". For the moment, though, she is full of enthusiasm, having just returned from Capri and Naples with a slew of new recipes.

"They did this gorgeous pasta dish with yellow zucchini flowers," she gushes. "So I had a word in the kitchen...". It will be appearing on the menu today, complete with mussels. She smiles. You can tell Ms Gray can already taste it on her tongue. She has a library, a database, a whole government research facility of tastes and flavours in her head. "And we've got this hot olive sauce thing today which is just fackin' mayyja," she continues in an off-the-cuff impersonation of Jamie Oliver, the Cafe's glamorous young alumnus. Then someone arrives with the shelled peas. "Oh no, those are brutes," she says. "Someone will have to pick out the small ones."

Ruth, though known to all as "Ruthie", is also Lady Rogers, the American second wife of Richard, the country's most dizzyingly eminent architect. A hearty welcomer, who seems to know every dweller in Media Gulch and Parliament Square, she is disappointed with the weather today. It has delayed the arrival of the English asparagus, which should come into season at the end of April. Her light blue eyes look hurt. The restaurant will be serving Spanish ones today, but, you know...

A steel jar of Cypriot courgettes is brought for the girls' inspection. "Aaarggh!" they cry in unison. For the courgettes, while as fat and bulbous as grenades, are pathetically, sheepishly, pale.

Theo Randall, who began his career with Chez Panisse in California, fields all their suggestions, adds his own, reports on the useable leftovers ("We had a perfect salmon last night, and only one potion sold") and holds it all together. He knows what can and can't be done. He and Ruth bandy ingredients for the mozzarella dish across the table: "Very finely sliced lemon...", "with rocket and olive oil...", "with a tiny bit of dried chilli, and salt and pepper...", "and marjoram, which is everywhere now". He writes everything down with an It-will-be-done flourish.

They draw up what-to-cook rosters for the seven chefs and the two proprietors. "Who's good at blanching broad beans?" asks Rose. The question sounds impossibly pretentious. But no, this is the currency of kitchen language. "There's a real art to it," says Ruth. "It's all about concentration."

By 9.45am, the first tasks are assigned. Someone is dispatched to peel the potatoes, someone else to stone the olives, and someone else again to fix Rose's blasted car alarm, which has taken to shrilling uncontrollably while she's driving...

10.45am. Everyone has been "prepping", as in preparing vegetables, energetically for half an hour, the Victorinox kitchen knives flashing and dipping through skinny leeks, red onions and an obscure Italian green called agritti. In Theo's hands, the chopping action is so fast, it's like a one-hand drum roll. The steel counter becomes a Mondrian abstract of different coloured boards - brown for legumes, blue for fish, red for meat, white for dairy products. In the kitchen, Grant from Melbourne has the fiddliest task in the world, which is filleting anchovies without breaking their tiny, pungent grey hides. Matthew is spreading a mixture of mascarpone, thyme and lemon into the secret crannies of a dozen guinea fowl. Deborah is pestling garlic and sage in a mortar. Everything is rising on a warm thermal of concentration.

Theo is directing operations. The business of cooking, you learn from watching him, is about courage and resolution. You can choose the best ingredients in the world, slice them with Japanese precision into matchstick proportions - and still won't get anywhere unless you've got a Theo to be bold and resolute, put it on the flames and take it off at the right time, season it with confidence, pour that anchovy-cream mix over the leeks and potatoes at the right consistency... He is probably the only chap in chef's whites from whom the girls would willingly take orders.

Ruth is making minestrone with broad beans. She's cooking chopped-up onions, carrots and celery in a saucepan. "It's the base of the soup," she says. "They melt into the one vegetable. It's called a soffrito." "Is that the same as a mirepoix?" I ventured, sotto voce. "John," she whispered back. "Don't bring that Frenchified nonsense in here." It was not the only time I was ticked off. I asked why they cooked asparagus in a saucepan, not a special kettle. Hah! That was only for ignorant, tree-trunk-sized asparagus, and who'd want to eat that? These golfball-sized artichokes, from which Amy was discarding the outer leaves - weren't you supposed to serve the leaves up to be eaten with, you know, melted butter? Oh come on. Which slum were you born in?

Rose is making the pasta dish with mussels. She tends to bang her pots, like a chef from Central Casting, and glare into her pans. Anxious to placate this awesome dominatrix, the mussels burst from their shells in saffron-coloured glee as if performing the "Halleleujah Chorus". Rose throws some wine and basil into their black-shelled ranks like a prize. She is always talking food, taking decisions, hectoring waiters, explaining things... "These small tomatoes are called Vesuvio because of the pointy bit on the bottom. The best home-made ravioli is the half-moon-shaped pumpkin pansoti which I found in Liguria..."

Ruth is more dreamy and chats about music and parties. She loves parties. "I love the way people are going for bigger and better events all the time," she says. "More parties, that's what we need." She believes in letting food cook for ages, and leaves her minestrone over a roasting flame for what seems hours.

12 noon. A flurry of activity as the fish arrives from suppliers in Tonbridge Wells. It includes a massive turbot with the face of an elderly jazz trumpeter, and the size and colour of a Stonehenge slab. It's been caught the previous night and is, as they say in Cork, "leppin' fresh". Theo attacks it with a vast theatrical dagger, wrestles its head off, hoicks out the unspeakable guts and, with the aid of a hammer, starts to cut it into 18 thick tranches. He holds one up to my face. "Sniff," he says. "Doesn't smell of fish. Just the sea." Nearby, Santos, a moustachioed gaucho from Sao Paolo, is preparing squid - slicing, wiping, washing, scoring, rolling. "He's the best squid prepper in the UK," murmurs Theo. "We'll get through three stone of it today". A dismaying sinkful of rubbery suckers awaits the Brazilian maestro.

12.20. The Menu Meeting. Rose gathers the waiters around her and takes them through the menu, explaining how every dish is done, what's meant by pagnotta bruschetta, and what zucchini flowers are doing in the tomato sauce. Frankie, an Irish part-timer from Sligo, calms his hunger by clamping some rocket leaves and a spoonful of chilli sauce between two slices of ciabatta bread - a very River Cafe thing to do.

12.30 pm. Around us, platefuls of cooked asparagus, wilted spinach, wafer-thin lemon and Swiss chard in its olive-green-and-cream livery, stand waiting to become River Cafe dishes. The first marinated lamb steaks slap on the grill. The first pans of guineafowl go into the wood-roasting oven. The air is thick with expectation. Among the first arrivals is Elizabeth Murdoch with what looks like an aspirant news reader, and Nick Rhodes from Duran Duran with what seem to be some teenage fans.

12.45 pm. Two of Ruth's daughters have arrived for lunch, and she goes to coo over her grand-daughter. Rose tucks a lock of straw-blonde hair behind her ear and surveys her kingdom with evident satisfaction. "That plate needs a wipe," she says to a passing waitress. "Here, let me do it." A flick of the wrist and the plate is on its way to the Sky TV boss. "So, John," she says, as a kind of afterthought. "Fancy some lunch?"

The River Cafe Cookbook Green is published by Ebury Press at £30. 'Independent' readers can order a copy for the special price of £25, including UK p&p, by calling TBS Direct on 01206 255800 and quoting this offer.