In a large bowl, combine the onion, saltines, egg white, milk, horseradish, salt if desired, and pepper. Crumble beef over mixture. Shape into 4 oval patties. , In a large skillet over medium heat, cook patties until no pink remains and a thermometer reads 160°, 5-6 minutes per side. , Remove patties and keep warm. Add the gravy, mushrooms and water to skillet; cook until heated through, 3-5 minutes. Serve with patties and noodles if desired.