In a large bowl, mix ground beef, breadcrumbs, egg, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Shape into 4–6 patties.
Heat a skillet over medium-high heat and lightly brown the patties for 2–3 minutes per side. This helps seal in the juices and enhances flavor.
Place sliced onions and mushrooms at the bottom of the slow cooker. Lay the seared patties on top, then pour beef broth over them. Cover and cook on low for 4–5 hours or until the patties are fully cooked and tender.
Once the patties are done, transfer them to a plate. Stir a cornstarch slurry (1 tablespoon cornstarch + 2 tablespoons water) into the slow cooker to thicken the gravy. Let it cook for 5 more minutes.
Return the patties to the slow cooker and coat them in the thickened gravy. Serve hot with mashed potatoes, rice, or buttered noodles.