2 small ready-cooked lobsters, about 675 g (1 lb 5 oz) each
50 g (2 oz) Gruyère cheese, grated
Heat the butter and oil in a small saucepan. Add the shallot and cook for 5 minutes until softened. Pour over the sherry and cook for 2 minutes until nearly boiled away. Stir in the mustard and crème fraîche, heat through and season with salt.
Meanwhile, using a large knife, cut the lobsters lengthways in half. Remove the meat from the tail and claws, reserving the main shell halves. Cut the lobster meat into large chunks.
Add the lobster meat to the sauce and warm through. Carefully spoon into the tail cavities of the reserved lobster shell halves and scatter over the Gruyère. Cook under a preheated hot grill for 3–5 minutes until golden and bubbling.