Trim the excess fat from the hearts and cut the dividing wall to make a cavity. Soak for 1 hour in cold water. Rinse well and drain. Mix the dry ingredients together and bind with egg and lemon juice. Pack the stuffing well into the hearts. Place in a 2 and a half pint oval casserole dish, pour over the stock and cover with a tight-fitting lid. Baste the hearts frequently, while cooking. Cook for 1 and a half hours at gas Niark 3, 325F (170C). In the last half-hour of cooking add the sherry. Serve piping hot with a selection of vegetables.