Prepare the Beef: Trim excess fat from chuck roast and season lightly with salt and pepper.
Optional Sear: Heat olive oil in a skillet, sear beef on all sides until browned. Transfer to crockpot.
Add Aromatics: Place onion slices and garlic in crockpot.
Pour in Liquids: Add beef broth, soy sauce, Worcestershire, balsamic vinegar, and Dijon mustard.
Add Herbs: Stir in thyme, rosemary, bay leaf, and black pepper.
Slow Cook: Low: 8–10 hours,High: 5–6 hours
Shred Beef: Remove beef, shred with two forks, and return to juices. Remove bay leaf.
Assemble Sandwiches: Pile beef onto rolls, top with provolone, and toast if desired. Serve with small bowls of au jus for dipping.