Place the beef chuck roast in the slow cooker along with the quartered onion, garlic, beef broth, Worcestershire sauce, thyme, salt, and pepper.
Cover and cook on low for 8 hours until the beef is very tender and shreds easily.
About 30 minutes before the beef is done, melt the butter in a skillet over medium-low heat. Add the sliced onions and cook slowly, stirring now and then, until they turn golden and soft.
Remove the beef from the slow cooker and shred it with two forks. Strain the cooking liquid and keep it warm as the au jus.
Split the hoagie rolls and divide the shredded beef and caramelized onions among them. Top each sandwich with two slices of provolone cheese.
Place the sandwiches under the broiler for 1 to 2 minutes until the cheese melts. Serve right away with small bowls of the warm au jus for dipping.