Step 1: In a large bowl, combine the ground beef, breadcrumbs, egg, finely minced onion, ketchup, Dijon mustard, 1 tablespoon Worcestershire sauce, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Gently mix until just combined, being careful not to overmix. Divide the mixture into 4-6 equal portions and form into oval-shaped patties, about ¾-inch thick. Make a slight indentation in the center of each patty.
Step 2: Heat 1 tablespoon of olive oil in a large, heavy-bottomed skillet over medium-high heat. Add the Salisbury steak patties and sear for 3-4 minutes per side until beautifully browned. Remove the seared patties from the skillet and set aside on a plate.
Step 3: Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil (if needed), sliced mushrooms, and diced onion to the skillet. Cook, stirring occasionally, for 5-7 minutes, until the mushrooms are softened and the onions are translucent.
Step 4: Sprinkle the 3 tablespoons of flour over the cooked mushrooms and onions. Stir well, scraping up any browned bits from the bottom of the pan, and cook for 1-2 minutes to cook out the raw flour taste.
Step 5: Gradually pour in the 2 cups of beef broth and remaining 1 tablespoon of Worcestershire sauce, whisking constantly to prevent lumps. Bring the mixture to a simmer, continuing to whisk until the gravy begins to thicken. Season with salt and pepper to taste.
Step 6: Return the seared Salisbury steak patties to the skillet, nestling them into the simmering gravy. Reduce the heat to low, cover the skillet, and let it simmer for 10-15 minutes, or until the patties are cooked through and the internal temperature reaches 160°F (71°C).
Step 7: Remove from heat and garnish with fresh parsley, if desired. Serve your classic Salisbury steak recipe immediately with your favorite comforting sides.