With sharp knife, make 6 evenly spaced slits deep into beef roast. Insert garlic into slits. Sprinkle pepper evenly over entire roast; rub pepper into roast.
Spray 3- to 4-quart slow cooker with cooking spray. Place onion slices in slow cooker; pour broth over onion. Place roast over onion and broth.
Cover; cook on Low setting 6 to 8 hours.
Remove roast from slow cooker; place on cutting board. Cut roast across grain into thin slices; return slices to slow cooker to moisten. Fill each toasted roll with beef, onion and 1 tomato slice. If desired, spoon small amount of broth from slow cooker over beef.