Blend mushroom soup, mustard, Worcestershire sauce, and horseradish (if using) in a bowl.

In a separate bowl, combine ground beef, 1/4 cup of the soup mixture, egg, bread crumbs, minced onion, salt, and pepper.

Shape the mixture into patties.

Heat a skillet over medium heat and brown the patties on both sides.

Pour off any excess fat from the skillet.

Stir in the remaining soup mixture, water (about 1/4 cup), and a pinch of parsley flakes into the skillet.

Bring the mixture to a simmer, then cover and cook over low heat for 20 minutes, or until the patties are cooked through.