Sprinkle ribs with salt and pepper. Place onion in a 6-qt. slow cooker. Top with ribs. In a large bowl, combine barbecue sauce, maple syrup and mustard; pour over ribs. Cook, covered, on low 6-8 hours or until meat is tender., Transfer meat to a serving platter; keep warm. Pour cooking liquid into a large saucepan; bring to a boil. Reduce heat; simmer, uncovered, 10 minutes or until sauce is thickened. Serve with pork. If desired, sprinkle with onions.