Step 1

In a large bowl, beat the egg; stir in onions, bread crumbs and mustard. Add beef and mix lightly but thoroughly. Shape into four 1/2-in.-thick patties.

Step 2

In an ungreased skillet, cook patties until meat is no longer pink, 4-5 minutes on each side; drain.

Step 3

In a small bowl, combine gravy, water and horseradish; add mushrooms. Pour over patties. Cook, uncovered, until mushrooms are tender and heated through, about 5 minutes. If desired, sprinkle with parsley.