In a large bowl, mix together ground beef, egg, bread crumbs, pepper, salt, onion powder, garlic powder, and Worcestershire sauce. Form into 5 balls, and flatten into patties.
Heat oil in a large skillet over medium heat. Fry the patties and onion in the oil until patties are nicely browned, about 4 minutes per side. Remove patties to a plate, and keep warm.
Saute onion and mushrooms in hamburger skillet until onions are transparent. Sprinkle flour over the mushroom-onion mixture. Stir in with a fork. Gradually mix in beef broth and sherry. Season with seasoned salt. Simmer and stir over medium-low heat, scrapping the bottom, until gravy thickens. Turn heat to low, return patties to the gravy, cover and simmer for another 10 minutes.