Step 1: In a large bowl, gently combine the ground beef, finely diced yellow onion, breadcrumbs, egg, milk, 1 tablespoon Worcestershire sauce, garlic powder, onion powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix until just combined, being careful not to overmix, which can lead to tough patties.
Step 2: Divide the mixture into four equal portions. Form each portion into an oval-shaped patty, about 3/4-inch thick. Press gently to keep them compact.
Step 3: Heat 1 tablespoon of olive oil in a large, heavy-bottomed skillet (such as cast iron) over medium-high heat. Once shimmering, carefully place the patties in the hot skillet. Sear for 3-4 minutes per side, until a deep brown crust forms. Do not cook through. Remove the seared patties from the skillet and set aside on a plate.
Step 4: Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil and 1 tablespoon of butter to the same skillet. Add the sliced mushrooms and cook, stirring occasionally, until they are tender and browned, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 5: Sprinkle the flour over the mushrooms and garlic. Cook, stirring constantly, for 1-2 minutes to cook out the raw flour taste, creating a roux.
Step 6: Slowly whisk in the beef broth and the remaining 1 tablespoon of Worcestershire sauce. Scrape up any browned bits from the bottom of the pan to incorporate all the flavor into the gravy. Bring the mixture to a simmer, stirring frequently, until the gravy begins to thicken to your desired consistency.
Step 7: Return the seared Salisbury steak patties to the skillet, nestling them gently into the simmering gravy. Reduce the heat to low, cover the skillet, and let the steaks simmer for 15-20 minutes, or until they are cooked through and very tender. Stir the gravy occasionally.
Step 8: Taste the gravy and season with additional salt and black pepper if needed. Garnish with fresh chopped parsley before serving hot. Enjoy your Best Salisbury Steak Recipe!