Boil water and cook shells according to package instructions.
Melt butter in large frying pan. Add pork and beef and cook until lightly browned. Add onion, celery, and garlic. Cover and cook 10-15 min, until celery is tender. Remove from heat, stir in sherry, grated cheese, salt, and bread crumbs. Pour some sauce into the bottom of a 9.5 * 13 glass casserole dish. Stuff each shell with slightly cooled mixture. Arrange stuffed shells in casserole and top with the rest of the sauce. Sprinkle mozzarella cheese on top and bake at 375 for 30 minutes. (serves 4+)