If desired, cut ribs into serving-size pieces. In a large shallow roasting pan place the ribs with the bone sides 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 onion. Pour mixture over ribs. Bake, uncovered, for 30 minutes more or until ribs are tender, basting once with sauce. Pass sauce with ribs.