Spray inside of 5- to 6-quart slow cooker with cooking spray.
Remove inner skin from ribs if desired. Mix onion powder, mustard, red pepper, all spice, cinnamon and garlic powder; rub mixture into ribs. Cut ribs into 4-inch pieces. Layer ribs and onion in slow cooker. Pour water over ribs.
Cover and cook on low heat setting 8 to 9 hours or until ribs are tender. Remove ribs from slow cooker; drain and discard liquid.
Pour barbecue sauce into shallow bowl; dip ribs into sauce. Place ribs in slow cooker. Pour remaining sauce over ribs. Cover and cook on low-heat setting 1 hour.