In a large mixing bowl, combine ground beef, salt, pepper, garlic powder, ketchup, Worcestershire sauce, steak sauce, egg, and breadcrumbs. Mix until all of the ingredients are combined.
Form the meat mixture into 4 patties (8 ounces each). Make an oval out of each. Like an extended burger. Place the patties on a dish or baking sheet.
In a large nonstick skillet over medium-high heat, heat the oil. Add the meat patties when the oil is heated but not smoking. To brown on all sides, cook them for 3-4 minutes per side.
Turn off the heat in the skillet. Remove the patties with a plastic spatula and arrange them on a dish or pan. Cover them loosely with foil to keep them warm. Don't worry if they're not completely done; they'll continue cooking in the gravy.
Remove any extra fat from the skillet but do not wash it. To add flavor, we'd want to prepare the gravy with the drippings.
Return the skillet to medium heat and add the butter. When the butter is melted and foaming, add the onions and simmer for 3-4 minutes on low heat. Stir every now and again.
After that, add the mushrooms. Cook, stirring periodically, for 3-4 minutes.
Stir the flour into the mushroom mixture to incorporate. Cook for 1-2 minutes, stirring often, to cook away the raw flour flavor.
Stir in the beef broth, scraping any pieces from the bottom of the skillet carefully. Stir until the flour mixture is completely dissolved.
Return the Salisbury steak patties to the pan after the sauce has reached a simmer (approximately 4-5 minutes). Allow the liquid to return to a simmer before lowering the heat to medium low.
Cook for 20 minutes, stirring periodically. During the cooking time, flip the steaks and swirl the sauce 2-3 times. Maintain a simmer in the gravy, adjusting the heat as required.
Uncover the skillet and continue to cook for 7-10 minutes to thicken the gravy. Maintain a low heat. Adjust the temperature as needed. As it cools, the gravy will thicken. Don't go too far with that.
Use an instant read thermometer to ensure that the internal temperature of the biggest piece of steak is at least 160°F.
If necessary, season with salt. We did not add anything more to ours as a reference.