Step 1

In a large bowl, add the egg and beat it well.

Step 2

Add the green onions, the breadcrumbs, and the mustard to the beaten egg and stir to combine.

Step 3

Add the beef to the egg mixture and mix lightly but thoroughly.

Step 4

Shape the beef mixture into 4 (1/2-inch-thick) patties.

Step 5

In a skillet over medium-high heat, add the beef patties and cook, flipping them once, until the meat reaches an internal temperature of at least 145 degrees F for medium doneness, about 4-5 minutes per side.

Step 6

Drain the fat from the skillet.

Step 7

In a small bowl, add the gravy, the water, and the horseradish and stir to combine.

Step 8

Add the mushrooms to the gravy mixture and stir to combine.

Step 9

Pour the gravy mixture over the beef patties in the skillet and cook, uncovered, until the mushrooms are tender and heated through, about 5 minutes.

Step 10

Plate the patties over the mashed potatoes and top with the gravy.

Step 11

Garnish with the parsley and serve.