Layer the sliced onions across the bottom of your slow cooker to create a flavorful base. Place the beef roast directly on top of the onions, positioning it in the center for even cooking.

Pour the beef broth, Worcestershire sauce, and soy sauce over the meat. Sprinkle the garlic powder, onion powder, thyme, black pepper, and salt evenly across the surface. Cover with the lid and set to LOW for 8 hours or HIGH for 4-5 hours, until the beef pulls apart easily with a fork.

Remove the tender beef to a cutting board and let it rest for 5 minutes. Using two forks, shred the meat by pulling it apart along its natural grain. Strain the cooking liquid through a fine-mesh strainer to remove the onions, then return the broth to the slow cooker.

Create a slurry by whisking together the cornstarch and cold water until completely smooth with no lumps. Stir this mixture into the hot broth and let it cook on HIGH for 10-15 minutes, stirring occasionally until the gravy thickens enough to coat the back of a spoon.

While the gravy thickens, toast your bread slices until golden and slightly crisp. To assemble each serving, place one slice of toast on a plate, spread about 2/3 cup of warm mashed potatoes over it, top with a generous portion of shredded beef (about 1/6 of the total), and ladle plenty of hot gravy over everything. Serve immediately.