Duxelles is a paste made from mushrooms sautéed with shallots and herbs in butter — finely chopped mushrooms cooked down until they release and reabsorb their liquid, concentrating into a dark, intensely savoury mince. Named for the Marquis d’Uxelles, whose chef is credited with its invention, it is a workhorse of classical French cooking: spread under puff pastry in beef Wellington, stuffed into vegetables, folded through sauces, or used as a flavour base wherever mushroom depth is wanted without bulk. The long, slow cooking is the whole technique — rushing it leaves the mushrooms watery and bland. Duxelles keeps well refrigerated and freezes cleanly, which is why restaurant kitchens make it in batches. It is mushroom flavour distilled to a paste, one of the quiet foundations of French savoury cookery.