Place short ribs in slow-cooker. In medium bowl, combine mustard, garlic, honey, salt, pepper, and barbecue sauce. Pour over ribs. Cover and cook on LOW 6-7 hours or until tender.
Refridgerate several hours or overnight. Skim any solidified fat from top. Remove ribs; heat in microwave or conventional oven.
Dissolve cornstarch in water. Add to sauce. Cook and stir in microwave or stove-top until hot and slightly thickened. Pour hot sauce over warm ribs.