Caramelize the Onions: Melt the butter in a skillet over medium-high heat. Add the sliced onions and cook for 15–20 minutes, stirring frequently, until they are soft, rich, and golden brown. Stir in the minced garlic for the last 1 minute.
Season the Roast: Pat the chuck roast dry with paper towels and season all sides generously with salt, black pepper, and dried thyme.
Layer in the Slow Cooker: Place half of the caramelized onions at the bottom of the slow cooker. Lay the seasoned chuck roast on top of the onions, then spread the remaining onions over the meat.
Add the Liquids and Seasoning: In a small bowl, whisk together the beef broth and Worcestershire sauce, then pour it around the roast. Sprinkle the dry onion soup mix evenly over the top of the meat and onions.
Slow Cook: Cover and cook on LOW for 8 hours (or on HIGH for 4–5 hours), until the beef is fork-tender and easily shreds apart.
Shred and Serve: Shred the beef right in the slow cooker juices, mix with the tender onions, garnish with fresh parsley, and serve hot!
