Combine ground beef, bread crumbs, chopped onion, egg and salt and pepper in a bowl until mixed evenly. Shape meat mixture into 4 patties 3/4 inch thick.
Fry patties in a large skillet over medium heat until browned on both sides 10 minutes Add beef broth, sliced onion, and mushrooms bring to a boil. Reduce heat to low cover and simmer until patties are no longer pink in center
Transfer patties to a platter and keep warm.
Bring onion mushroom mixture to a boil ( mix cornstarch with cold water in a small bowl ) stir into onion mushroom mixture.
Cook and stir until gravy thickens 1 minute or so. Pour over patties and serve. Serve with mashed potatoes and peas or rice.