Cut cold lamb into neat, thin slices. Into a rather deep, broad frying-pan, put a tablespoonful of butter, a dash of cayenne, salt and pepper, a great spoonful of vinegar and the same of currant jelly, with a small teaspoonful of French mustard. Heat to boiling, keeping your spoon busy all the while until the ingredients are thoroughly incorporated. Then lay in the lamb and let them get smoking hot through. Lay upon a hot-water dish and pour the sauce over them.