It is after nine, that time of night when Londoners like to eat, and the dining room at Hibiscus, chef Claude Bosi's restaurant in Mayfair, is full. This used to be a tight-laced space of thick carpets and caramel wood veneer, but a recent redesign loosened its corset. There is a bare wood floor, the walls are in stylish shades of gray, and some clientele are clad in jeans. Few men wear neckties. Hibiscus may have two Michelin stars, and its Lyonnais-born chef's cooking may be serious, but the mood is not. In 2007, when he opened in London, Bosi made a splash with a fancy take on that classic British snack, the sausage roll, made with rare-breed pork and a pastry composed of so much butter it was a wonder it held together. He does not do po-faced. So here comes an egg carton labeled with the words "A little gift from the team at Hibiscus." Inside: Legbar chicken eggs with sky blue shells, their tops sliced to reveal a delicate foam of egg white flavored with coconut and curry powder and, beneath, a powerful royale of chestnut mushrooms with the texture of yolk. The sweetest of prawns from the Welsh coast are served raw in a warm beurre blanc made with smoked butter and dotted with pearls of caviar. It is salt and smoke and fat and the freshest seafood—everything I want to eat. Most strikingly, a sautéed lamb sweetbread the color of copper comes with a chilled sorrel-and-apple sauce the green of a spring lawn and with appropriately grassy edges to its flavor. I finish my meal with a mille-feuille of Agen prune and whiskey cream, with a puff pastry so flaky it may well contain the thousand leaves of its French name. Claude Bosi does like to deliver on a promise.