Wash and clean the livers of all tissue. Fry the onion lightly in 2 tablespoons butter. Add livers, salt and pepper and fry with onion for 5 minutes, then cover frying pan and simmer until liver is cooked through. Watch carefully that is does not become too dry. If this does happen, add the 1/4 cup water or stock. Put the mixture with 4 tablespoons butter through a grinder or blender until soft. If it seems too dry, add a little more of the cool water or stock. Pile the liver paste on a plate in a pyramid and garnish with the remaining 2 tablespoons butter, softened, forcing it through an icing tube.