In a large bowl, combine ground beef, breadcrumbs, egg, garlic powder, Worcestershire sauce, salt, and pepper. Mix until thoroughly combined, then shape into 4 oval patties.
In a large skillet over medium heat, melt butter. Add beef patties and cook until well browned on each side, about 4-5 minutes per side. Remove patties from skillet and set aside.
In the same skillet, add sliced onions and cook until soft and golden, about 5 minutes.
Sprinkle flour over the onions and stir to coat. Gradually whisk in beef broth, bringing to a simmer to thicken the gravy.
Return the beef patties to the skillet, spooning gravy over top. Reduce heat to low, cover, and cook for an additional 10 minutes to ensure patties are cooked through.
Serve the Salisbury steak warm with onion gravy poured over top. Enjoy!