Rub ribs with Cajun seasoning. Layer ribs and onion in a 5-quart slow cooker. In a bowl, combine the next 12 ingredients. Pour over ribs. Cover and cook on low for 6 hours or until meat is tender.
Remove ribs and keep warm. Strain cooking juices and skim fat; transfer to a saucepan. Combine cornstarch and water until smooth; stir into juices. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with ribs.