Place ribs in a 5-qt. slow cooker. In a small bowl, combine the onion, ketchup, brown sugar, vinegar, tomato paste, paprika, Worcestershire, mustard, salt and pepper; pour over ribs. Cover and cook on low for 5-6 hours or until meat is tender.
Remove ribs to a serving platter; keep warm. Skim fat from cooking juices; transfer juices to a small saucepan. Bring to a boil.
Combine cornstarch and water until smooth. Gradually stir into the pan. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with ribs.