Diner Salisbury Steak

1. In a medium bowl mix together the ground beef, scallions, bread crumbs, egg, dehydrated onions, garlic powder, and mustard and shape into four 1/2-inch-thick oval patties.
2. Preheat oven to 120° F.
3. Heat the oil in a large skillet over medium heat and cook the patties for 3 to 4 minutes per side, or until no pink remains. Remove patties from skillet to a paper towel lined platter to drain and place platter with the Salisbury steaks in oven to keep warm. Discard the oil in the pan and wipe clean.
4. Return the skillet to the heat and add the gravy, the tomato paste, water, horseradish, and mushrooms and cook for 4 to 5 minutes, or until the mushrooms are tender, stirring occasionally.
5. Return the warm steaks to the skillet, lower the heat to low and simmer for another 2 or 3 minutes.
6. To serve, place 1 steak, a serving of corn, and a serving of mashed potatoes on each of 4 plates and place 1/4 of the mushroom gravy over each steak. Serve immediately.