Toast the bread until crisp, then pulse or crush into fine breadcrumbs.
In a large bowl, combine ground beef, chopped onion, egg, breadcrumbs, Worcestershire sauce, garlic powder, salt, and pepper. Gently mix and form into 4 oval patties.
Heat a large nonstick skillet over medium heat. Cook patties for 4-5 minutes per side until browned and cooked through. Remove and set aside.
Without cleaning the pan, add the mushrooms and sliced onion. Cook for 6-8 minutes, stirring often, until softened.
Pour in beef broth, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Scrape up browned bits and simmer for 3-4 minutes.
Stir in cornstarch slurry and simmer 1-2 minutes until gravy thickens slightly.
Return patties to the skillet, spoon gravy over the top, and simmer for 2-3 minutes before serving.