Season the beef chuck roast with salt and freshly ground black pepper on all sides.
Heat olive oil in a skillet over medium-high heat and sear the roast for 3 to 4 minutes on each side until browned.
Place the sliced onions and minced garlic in the bottom of the slow cooker.
Put the seared roast on top of the onions and garlic.
Pour beef broth, soy sauce or Worcestershire sauce, Dijon mustard, dried thyme, and bay leaf around the roast.
Cover and cook on low for 8 to 10 hours, or on high for 4 to 6 hours, until the beef is very tender.
Remove the roast from the slow cooker and shred it using two forks.
Return the shredded beef to the slow cooker and mix it thoroughly with the juices.
Slice the rolls and fill them with the shredded beef. Add cheese if desired.
Serve the sandwiches with warm au jus on the side for dipping.