Cook the lima beans according to package directions. Drain, reserving about one-quarter cup of the cooking liquid. Rub the beans through a food mill or blend in an electric blender. If a blender is used, blend only a small quantity of beans at a time, adding a little of the reserved liquid as necessary and stirring down occasionally with a rubber spatula.
Transfer the purée to the top of a double boiler and keep hot. Add the cream and butter and season with black pepper to taste. The purée should have the consistency of mashed potatoes. Serve sprinkled with chopped parsley.