Put into a saucepan the butter and onion. Before it takes color add the flour, and cook, but not brown. Then add slowly one cupful of water in which the lobster was boiled, the milk, and stock. Add the lobster meat, and when it has become thoroughly hot remove the meat and place it on the dish on which it is to be served, arranging it in the shape of a lobster as far as possible. Cut the tail-piece into thick slices, without changing its position. Season the sauce with salt, pepper and cayenne, and pour it over the meat. Place around the edges triangular croutons, and garnish with head, small claws, and tail.