Mix ground beef, bread crumbs, salt, pepper and egg; shape into 4 oval patties, each about 2 cm thick. Cook patties in 10 in skillet over medium heat, turning occasionally, until brown, about ten minutes; drain. Add onion, broth and mushrooms. Heat to boliling, reduce heat and simmer until beef is done, about 10 minutes.
Remove patties, keep warm. Heat onion mixture to boiling. Mix water and cornstarch; stir into onion mixture. Boil and stir 1 minute serve over patties.