Place the sliced onion rings in the bottom of the slow cooker in an even layer.
Season the chuck roast with dried thyme, onion powder, and black pepper. Set the roast on top of the onions.
In a bowl, whisk together beef broth, soy sauce, and minced garlic. Pour the liquid around the roast.
Cover and cook on low for 8 to 10 hours or on high for 5 to 6 hours, until the beef shreds apart effortlessly with a fork.
Remove the roast and shred the meat using two forks. Return the shredded beef to the slow cooker and let it soak in the juices for 5 minutes.
Pile the shredded beef and cooked onions onto the toasted hoagie rolls and top each with a slice of provolone cheese. Place under the broiler for 1 to 2 minutes until the cheese is bubbly and melted.
Strain the remaining cooking liquid through a fine mesh strainer into small bowls for dipping. Serve the sandwiches warm alongside the au jus.