Place roast, onion, salt, black pepper, and broth in a 4-5 quart slow cooker. Cover and cook on Low for 6-8 hours or High for 4-6 hours.
Shred meat with 2 forks. Remove meat and onions from the slow cooker, then evenly divide among the rolls. Transfer broth to small bowls for dipping and serve with the sandwiches.