Whisk together all BBQ sauce ingredients from tomato paste to black pepper in a medium bowl until smooth.
Layer the sliced onions in the base of a 4-quart or larger slow cooker and place the chicken on top.
Pour the prepared BBQ sauce evenly over the chicken and onions.
Cover the slow cooker and let it cook on HIGH for 3-4 hours or on LOW for 5-6 hours until chicken is tender.
Carefully remove the chicken from the slow cooker and shred it using two forks or shredding tools.
Return the shredded chicken to the slow cooker and mix it well to coat with the sauce.
Keep the slow cooker on WARM until ready to serve the pulled chicken sandwiches.