If desired, cut ribs into serving-size pieces. In a large shallow roasting pan, place the ribs, bone side down. Bake, covered, in a 350 degree F oven for 1 hour. Carefully drain off liquid in roasting pan.
Meanwhile, in a medium bowl combine the ketchup, water, vinegar, Worcestershire sauce, paprika, chili powder, black pepper, salt, and cayenne pepper. Stir in onions. Pour mixture over ribs. Bake, uncovered for 30 minutes longer or until ribs are tender, spooning sauce over ribs once. Pass sauce with ribs. Makes 6 servings.