Dice onion into small chunks and add to the bottom of the slow cooker.
Add beef roast into slow cooker on top of onions.
In a bowl whisk together melted butter, beef broth, salt, pepper, onion powder, garlic powder, and balsamic vinegar.
Pour into slow cooker over the roast and cover with a lid.
Cook on LOW for 8 hours or on HIGH for 5 hours. Don’t open the lid during this cook time.
When finished cooking, take beef out of the slow cooker and shred it. Should be easy to do with forks.
Place shredded beef back into the slow cooker full of au jus. Let beef sit in the au jus, covered with the lid, for 5-10 minutes to moisten and become full of flavor or until you're ready to serve.
After 5 minutes, strain au jus out of the beef and onions and serve in small bowls for dipping the sandwiches.
If you haven’t yet, cut French bread down into smaller rolls and slice each one 3/4 of the way through horizontally. Place shredded beef inside and a slice of provolone cheese.
Serve sandwiches on a plate with a bowl of au jus for dipping. Enjoy!