In a large mixing bowl, beat one large egg until frothy. Add in sliced green onions, seasoned bread crumbs, and prepared mustard, mixing until combined. Gently fold in the ground beef, shaping into four patties about ½-inch thick.

Preheat an ungreased skillet over medium heat. Cook the patties for 4 to 5 minutes on each side until golden brown and cooked through. Drain excess fat.

In a small bowl, combine beef gravy, water, and prepared horseradish if desired. Stir in sliced mushrooms and mix thoroughly.

Pour the gravy mixture over cooked patties in the skillet. Simmer uncovered for about 5 minutes until flavors meld.

Remove from heat and sprinkle with minced parsley if desired. Serve hot with mashed potatoes or egg noodles.