Season the beef: Pat the chuck roast dry.
Sprinkle all sides with salt, pepper, garlic powder, and onion powder. Press the seasonings in so they stick.
Sear for extra flavor (optional but recommended): Heat oil in a large skillet over medium-high. Sear the roast 2 to 3 minutes per side until well browned.
Transfer to the slow cooker.
Build the base: Add sliced onions and minced garlic to the slow cooker. Pour in beef broth, soy sauce, Worcestershire sauce, and balsamic vinegar. Stir in thyme, oregano, rosemary, and the bay leaf.
Slow cook: Cover and cook on Low for 8 to 10 hours, or on High for 4 to 5 hours, until the beef is fork-tender and shreds easily.
Shred the beef: Transfer the roast to a cutting board.
Discard large fat pieces. Shred with two forks into bite-size pieces.
Skim and season the jus: Use a ladle to skim excess fat from the surface of the cooking liquid. Taste and adjust with a pinch of salt or more pepper if needed.
Remove the bay leaf.
Return beef to jus: Add the shredded beef back to the slow cooker to soak up flavor. Keep on Warm while you prep the rolls.
Toast the rolls: Split the rolls and brush cut sides with melted butter. Broil cut-side up for 1 to 2 minutes until toasted and golden.
Watch closely to prevent burning.
Assemble the sandwiches: Pile warm shredded beef onto each roll. Top with provolone or Swiss. Broil again for 30 to 60 seconds to melt the cheese, if you like.
Serve with au jus: Ladle hot jus into small bowls for dipping.
Garnish sandwiches with parsley if desired. Serve immediately.