Place onion in a 3-qt. slow cooker; top with ribs. Combine the ketchup, plum sauce, chili sauce, brown sugar, celery seed, garlic powder, Liquid Smoke if desired and allspice; pour over ribs.
Cover and cook on low for 6-7 hours or until meat is tender. Shred meat with two forks and return to the slow cooker. Cover and cook 15 minutes longer or until heated through. Serve on rolls with additional chili sauce on each sandwich. Yield: 8 servings.