Place the chuck roast in the slow cooker. Top with the au jus seasoning, beef broth, and sliced onion (if using).

Cover and cook on low for 8 hours, or until the meat is fork-tender.

Preheat your oven's broiler. Place the hoagie rolls on a baking sheet, open them, and add a slice of provolone cheese to each roll. Broil for about 2 minutes, or until golden brown.

Shred the cooked roast with two forks.

Fill each hoagie roll with the shredded beef and serve.