Spray slow cooker with non-stick spray.
Lay down sliced onions on the bottom of the slow cooker (optional). Lay chicken breast over top of the onions.
Mix the BBQ sauce, chili sauce, brown sugar, apple cider vinegar, liquid smoke and all seasonings in a bowl. Spread mixture over top of the chicken breasts.
Cook on low for 6-7 hours, or on high for 3-4 hours. 30 minutes before the cooking is finished, use two forks to shred the chicken apart. Mix well with the sauce and allow to cook for another 30 minutes.
Butter the brioche buns, and grill the inside of each bun until golden brown (optional, but it adds an extra layer of flavor). Add pulled chicken, and a handful of coleslaw to the buns. Serve immediately.