Prepare barbecue sauce: In a medium bowl, whisk together first 11 ingredients, (ketchup through black pepper) until smooth; set aside.
In a crock cooker, place chopped onion and chicken; add reserved barbecue sauce and stir well to thoroughly coat chicken.
Cover and cook on LOW for 6 to 8 hours. Shred chicken with 2 forks then blend with crock cooker juices. Fill burger buns with shredded chicken mixture.