Place meat in slow cooker; top with onions and barbecue sauce. Cover with lid. Cook on LOW 8 to 10 hours (or on HIGH 4 to 5 hours).
Remove meat from slow cooker; cut off and discard excess fat. Chop meat into small pieces or shred with fork. Return to slow cooker; stir to coat with sauce.
Fill rolls with meat mixture and Singles just before serving.