Lobster Thermidor

Bring a large pot of well-salted water to a rolling boil. Plunge the lobsters in head-first and cook just until the shells turn bright scarlet and the meat is barely set — about 6 minutes for 600 g birds, no more. Lift them out and shock briefly in iced water to halt the cooking. Twist off the tails and claws, split the bodies lengthwise, and reserve the empty half-shells for serving.

Melt the butter in a wide sauté pan over medium-low heat. Add the minced shallots with a pinch of salt and sweat them gently, stirring, until soft, glassy and translucent without taking on any colour, about 4-5 minutes. This sweet, mellow base carries the whole sauce.

Pour in the cognac and, off the flame or with great care, ignite it to burn off the raw alcohol — let the flames die on their own. Add the white wine, raise the heat, and reduce the liquid by about two-thirds until it is syrupy and barely coats the pan floor, concentrating the shallot and brandy into a savoury glaze.

Pour in the cream and whisk in the Dijon mustard, a pinch of cayenne and white pepper. Simmer gently, stirring, until the sauce thickens enough to coat the back of a spoon, 4-6 minutes. Taste and season — it should be rich, faintly sharp with mustard, and warm with cayenne. Do not let it boil hard, or it may split.

Off the heat, whisk a spoonful of the hot sauce into the egg yolk to temper it, then stir the yolk back into the pan to enrich and bind — never let it boil now. Fold in the lobster meat and chopped tarragon, warming through for a minute. Spoon the mixture generously into the reserved half-shells set on a baking tray.

Scatter the grated Gruyère thickly over each filled shell. Slide under a hot grill (broiler) about 10 cm from the element and cook until the cheese melts, bubbles and blisters into golden-brown patches, 3-5 minutes. Watch constantly — it goes from pale to perfect to burnt in seconds. Serve at once, scalding hot, with lemon and crusty bread.