Lay half of the onion slices in the bottom of the slow cooker. Sprinkle with ½ cup of brown sugar, then top with chicken pieces. Cover with remaining onion slices then pour the Bar-B-Q sauce over the top. Finish with remaining ½ cup brown sugar. Cook on high for 5 hours.
When done pull the meat off the bone and shred. Serve with additional Bar-B-Q sauce (bun optional).