Spray 4- to 5-quart slow cooker with cooking spray. In cooker, place onion and garlic.
If beef roast comes in netting or is tied, do not remove; place beef on onion. Pour dressing over beef.
Cover; cook on Low heat setting 8 to 10 hours.
Transfer beef to cutting board; remove netting if necessary. Cut beef into thin slices; return to cooker. Stir in pepperoncini. Increase heat setting to High. Cover; cook 15 minutes longer or until thoroughly heated.
Spoon beef into toasted buns; top with bell pepper rings and cheese. Pour remaining liquid into 8 small custard cups; serve with sandwiches for dipping.