Step 1: In a large bowl, combine the ground beef, breadcrumbs, egg, 1 tablespoon Dijon mustard, 1 tablespoon Worcestershire sauce, minced onion, garlic powder, salt, and black pepper. Mix gently but thoroughly with your hands until just combined. Do not overmix.
Step 2: Divide the mixture into 4-6 equal portions and form them into oval-shaped patties, about 3/4 inch thick. Make a slight indentation in the center of each patty with your thumb.
Step 3: Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Carefully place the patties in the skillet and cook for 4-5 minutes per side, or until beautifully browned. They don’t need to be fully cooked through. Remove the patties and set aside.
Step 4: Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil to the same skillet. Add the sliced mushrooms and cook, stirring occasionally, for 5-7 minutes until softened and their liquid has released. (Cook in batches if necessary to brown them properly).
Step 5: Add the sliced yellow onion to the skillet with the mushrooms and cook for another 3-4 minutes until softened and translucent.
Step 6: Sprinkle the flour over the mushrooms and onions. Cook, stirring constantly, for 1-2 minutes to cook out the raw flour taste.
Step 7: Slowly whisk in the beef broth, scraping up any browned bits from the bottom of the skillet. Continue whisking until the gravy begins to thicken and smooth out, about 3-5 minutes. Stir in the additional 1 tablespoon Worcestershire sauce, 1 teaspoon Dijon mustard, and the optional soy sauce.
Step 8: Return the browned 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 15-20 minutes, or until the patties are cooked through and tender, and the gravy has thickened to your desired consistency.
Step 9: Taste the gravy and adjust seasonings if needed. Serve hot, garnished with fresh chopped parsley, alongside your favorite mashed potatoes or rice.