Start by seasoning the chuck roast generously with salt and black pepper. This step ensures the beef is well-flavored from the inside out. Heat a large skillet over medium-high heat, and sear the beef on all sides until browned. This seals in juices and adds extra depth to the flavor.
Transfer the seared beef to your slow cooker. Add the beef broth, water, soy sauce, Worcestershire sauce, minced garlic, sliced onions, thyme, and pepper. Gently stir the liquids to combine, then cover the slow cooker and set it on low for 6 hours or high for 4 hours.
Once the beef is tender and cooked through, remove it from the slow cooker and place it on a cutting board. Use two forks to gently shred the meat into smaller pieces. Return the shredded beef to the slow cooker to soak up more of the flavorful juices.
One of the key elements of a French Dip Sandwich is the au jus sauce. If you'd like to elevate the sauce further, remove 1 to 2 cups of the cooking liquid, skim off any excess fat, and simmer it in a saucepan for 5 minutes. Taste and adjust the seasoning with salt or additional Worcestershire sauce as needed.
Slice the hoagie rolls in half and lay them on a baking sheet. Lightly toast them under a broiler for about 2 minutes until golden and crispy. Add generous portions of shredded beef to the bottom half of each roll, top with provolone or Swiss cheese slices (if using), then close the sandwiches with the top half of the bread.
Serve the French Dip Sandwiches hot with individual bowls of warm au jus sauce for dipping. To garnish, sprinkle chopped parsley over the top, giving the sandwich a fresh touch.