To make the sandwich

• Cut roast into 4-inch chunks and generously rub salt and pepper all over the roast.
• In a gallon-size freezer bag, combine broth, onion, soy sauce, Worcestershire sauce, mustard, garlic and bay leaves. Add roast and seal. Marinate beef in refrigerator for at least 1 hour, or up to 12 hours.
• Pour contents of bag into a large slow cooker. Cook on low for 7 hours, until beef is tender.
• Remove roast, reserving au jus left in slow cooker. Discard bay leaves. Transfer roast to a cutting board and use two forks to shred beef.
• Preheat broiler. Slice deli rolls in half, place on a baking sheet lined with aluminum foil, and spray with a little cooking spray. Broil for 1 minute or until golden and toasted.
• Remove from oven and place a generous amount of shredded meat on bottom half of roll and add a slice of provolone cheese. Broil for 30 seconds more to melt cheese. Remove from oven and cover with top half of roll.
• Skim any fat off top of au jus in slow cooker and ladle liquid into small bowls. Serve sandwiches with au jus on the side for dipping, and some of the cooked onions, if desired.