Sear the chuck roast in a skillet with olive oil over medium-high heat until browned, then transfer to the slow cooker.
Sauté the sliced onions briefly in the same skillet before adding them to the slow cooker.
Pour the beef broth, soy sauce, Worcestershire sauce, minced garlic, thyme, and pepper over the meat.
Cook on low for 8 to 9 hours until the beef is tender enough to shred with a fork.
Shred the meat, return it to the broth, then assemble on toasted rolls with cheese and serve with the broth for dipping.