Layer the ingredients. Place the sliced onions evenly across the bottom of the slow cooker. Place the chuck roast directly on top of the onions.
Mix the broth. In a bowl, whisk together the beef broth, soy sauce, Worcestershire sauce, garlic powder, onion powder, thyme, and black pepper. Pour the mixture over the roast.
Cook. Cover and cook on low for 8 to 9 hours, or on high for 4 to 5 hours, until the meat is completely tender and pulls apart easily.
Shred the beef. Remove the roast to a cutting board and use two forks to shred the meat, discarding any large pieces of fat. Return the shredded beef to the slow cooker and let it soak in the au jus for 10 minutes.
Assemble and serve. Split the rolls, toast them briefly under the broiler, and load them with the shredded beef and onions. Top with cheese, broil until melted, and serve with small bowls of the au jus for dipping.
