1 Make the beef patties: In a mixing bowl, use your hands to mix together the ground beef, breadcrumbs, egg, salt, and pepper.
Divide the mixture into four equal pieces, then use your hands to gently shape each piece into a 1/2-inch-thick patty.
2 Sear the patties in a skillet: Heat the oil in a large skillet over medium heat. When the oil is shimmering, sear the patties on one side for 3 minutes, then flip them with a thin, flexible spatula. Sear them on the other side for another 3 minutes.
Transfer the patties to a plate.
3 Cook the mushrooms, onions, and garlic: There should be a few teaspoons of fat left in the pan from cooking the patties, but add additional oil if needed. Still over medium heat, add the mushrooms, onions, and garlic to the skillet.
Cook, stirring often, until the mushrooms have given up their liquid and just begun to brown, 6 to 8 minutes.
4 Stir in the liquids and braise the patties: Stir the tomato paste, mustard, and beef broth into the mushrooms and onions. Use a spoon to scrape up any brown bits from the skillet.
Return the patties to the skillet, spooning a bit of the cooking liquid, mushrooms, and onions over each one. Cover the skillet turn the heat down to low. Let the patties braise in the covered skillet for 30 minutes.
While the patties are simmering, you can boil a pot of noodles, make mashed potatoes, and/or prepare any other vegetable quick side dishes.
5 Thicken the cooking liquid into a gravy: Uncover the skillet and use a spoon to nudge the patties to the side of the skillet. In a small bowl, stir together the cornstarch and water. Stir in the cornstarch mixture and let it boil for about a minute, until the gravy has thickened, then turn off the heat.
6 Serve the Salisbury steaks: Transfer the patties to plates and ladle the gravy over top. Sprinkle with chopped parsley, if you like, and serve hot with egg noodles or mashed potatoes alongside.