Place pork, onions, apple slices,and bay leaf in slow cooker. Whisk together broth, half the barbecue sauce, tomato paste, flour, chili powder, Worcestershire-type sauce, garlic and pepper. Pour into slow cooker. Cover and cook on LOW for 8 hours or until pork is tender.
Remove any excess fat that has risen to the top and discard. Remove pork to a cutting board, cover, and let stand 10 minutes. Transfer sauce to a saucepan and bring to a boil over medium-high heat. Reduce heat and simmer until thickened, about 15 minutes. Add remaining barbecue sauce and heat through. Remove bay leaf.
Shred pork with 2 forks and combine with sauce. Serve on brioche buns with coleslaw on the side.