‘Well,” understated my friend as we embarked on a four-hour lunch at the most ravenously anticipated “pop-up” Britain has known, “it’s not much like the last one, is it?”

Indeed not. The temporary restaurant he had in mind had the novel address “Level 10, Peckham Multistorey Car Park”. From parking, it took us another hour to locate the roof, the lift having given up the ghost at level six. When finally we stumbled on the relevant hut, a Trustafarian kid greeted us with a breezy, “We were really busy last night, guys, so we’re not serving food.”

Thomas Keller of The French Laundry. Photo: The French Laundry

Unforgettable as the ensuing ersatz picnic, grudgingly served to us after sobbed entreaties, was, lunch at The French Laundry on the first of its 10 days of popping up at Harrods may yet prove still more amnesia-resistant.

On two counts, this is an unusual review. I cannot recommend a visit because Thomas Keller – an American chef as globally fabled as Heston Blumenthal and Ferran Adrià – will be jetting back to California, with napery, cutlery and three dozen staff, after his final gig on Monday night. Meanwhile, the usual critical criteria seem inadequate for a lunch that leads us towards the realms of the moral and philosophical. Can it ever be right to pay £250 per head for food alone, I ask you, and can any meal be worth that sum?

Whatever the answers, there is no temptation to make a cheap pun about The French Laundry taking us to the cleaners. This will shock no one familiar with the reputation of a restaurant known to Anthony Bourdain, god of all foodies, as “the world’s best restaurant, period”. Seldom will you encounter food as exquisite.

Even in a section cordoned off from Harrods’s Georgian Restaurant, intended to replicate the three-starred Napa Valley original but in fact evoking nothing so much as an unusually elegant corporate entertainment tent, Bourdain’s passion made sense. The service alone was astounding. One chap lectured us masterfully on the olive oil’s chemical composition (Keller is frighteningly obsessed with ingredients). Another, an enchanting young Liverpudlian, handled my cretinous Scouser gags with miraculous tolerance. Nothing (and God knows I tried) could have fazed them. Had I gone into renal failure, the sous-chef would have rustled up a dialysis machine en croute.

The French Laundry’s cornet of marinated Atlantic salmon. Photo: The French Laundry

In fact, the only potential medical crisis was occasioned by the spectacular breads and amuses-bouche. My friend mentioned an allergy to oysters, so his version of the Keller signature dish “oysters and pearls”, a sabayon of pearl tapioca with tiny chunks of Maldon oyster topped with white sturgeon caviar, came with extra caviar. This privation he bravely bore, and on balance the textural and taste contrasts which the oyster lent this subtly saline masterpiece were probably not worth a bout of anaphylactic shock.

The “oyster and pearls” dish from The French Laundry. The French Laundry

The next two dishes – a colourful salad starring Hawaiian peach palm hearts; and a chowder headlined by sturgeon – were merely delectable. The two after that were staggeringly brilliant.

“I had no idea it could taste like this,” he said of Maine lobster poached in emulsified Vermont butter. Keller has invented his own technique with this crustacean, and it was instantly clear that every other lobster we had met had been criminally overcooked. “That,” said my friend, “is the Platonic ideal of lobsteriness. Indescribably good.”

So was poularde en brioche, in which meat from a young, cereal-fattened hen was yoked to Tokyo turnip, green apple and Périgord truffle, the ensemble garnished with shavings of duck foie gras. “This is food gone bonkers. Unimprovable.”

Ordinarily, at such a point in a nine-course tasting menu, I toy with an off-menu request for an emetic in order to make room for the rest. Yet Keller’s touch with even the richest ingredients is so preposterously light that transient bulimia was never an issue.

The French Laundry “coffee and doughnuts” to finish. Photo: The French Laundry

A crimson piece of braised brisket was subtle enough almost to qualify as an unlikely palate cleanser. Slivers of Pleasant Ridge Reserve, a Wisconsin cheddar, were fabulously creamy and nutty, and came with figs and the cleverest, weeniest take on the (Ibérico) ham and cheese toastie. A huckleberry muffin (no Mark Twain pun apparently intended) came with a gloriously intense huckleberry sorbet. And a quintet of impossibly cute mini-puds gave way to “coffee and doughnuts”, in which the airiest, fluffiest beignets came with a sensational cappuccino semifreddo.

“TWO HUNDRED AND FIFTY QUID!” exploded my friend over petits fours styled after cinema treats (sweets with the flavours of root beer, potato crisps etc). “Sorry, but I’ve just seen it on the menu again. When we sat down, I thought the only way we could decide it was worth it was through drink. But we’ve not had much and, well, look, this is something I couldn’t imagine ever saying four hours ago, but I officially forgive you for the Peckham car park.”

One of the meals of my life, a debt of honour paid, a friendship salvaged… I may be no culinary-moral philosopher, but who could be so chillingly mercenary as to put a price on that