Spray 3- to 4-quart slow cooker with cooking spray. Mix onion, barbecue sauce, dressing and broth in slow cooker. Add beef; stir to coat.
Cover; cook on Low heat setting 8 to 10 hours.
Remove beef from slow cooker; skim fat from sauce. Shred beef with 2 forks; stir beef back into sauce. Using slotted spoon, place 1/2 cup beef mixture onto bottom of each bun. Cover with tops of buns.