In medium bowl, combine ground beef, onion, cracker crumbs, egg white, milk, horseradish, salt and pepper. Mix lightly but thoroughly, shape into 4 oval 1/2-inch thick patties.
Heat large nonstick skillet over medium heat until hot. Place beef patties in skillet. Cook 7-8 minutes, turning once or until centers are no longer pink and juices run clear. Center of patty should register 160 on an instant read thermometer. Remove from skillet and keep warm.
In same skillet, combine gravy, mushrooms, and water. Cook over medium heat 3-5 minutes or until mushrooms are tender.
Serve gravy over Salisbury Steak and mashed potatoes if desired.