Step 1: In a large bowl, combine the ground beef, finely chopped 1/4 of the onion, breadcrumbs, egg, milk, 1 tablespoon of Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Use your hands to gently mix until just combined. Overmixing can lead to tough patties.
Step 2: Divide the mixture into 4 equal portions and shape them into oval-shaped patties, about 3/4-inch thick. Create a slight indentation in the center of each patty with your thumb to prevent them from bulging while cooking.
Step 3: Heat 1 tablespoon of olive oil or butter in a large, heavy-bottomed skillet or Dutch oven over medium-high heat.
Step 4: Once hot, carefully place the patties in the skillet. Sear for 3-4 minutes per side, until a deep brown crust forms. The patties don't need to be cooked through at this stage. Remove the seared patties from the skillet and set them aside on a plate.
Step 5: Add the remaining 1 tablespoon of olive oil or butter to the same skillet. Add the sliced mushrooms and cook over medium heat for 5-7 minutes, stirring occasionally, until they are browned and have released their liquid.
Step 6: Stir in the remaining 1/4 cup of chopped onion and cook for another 3-4 minutes until softened.
Step 7: Sprinkle the flour over the mushrooms and onions. Cook, stirring constantly, for 1-2 minutes to create a roux.
Step 8: Gradually whisk in the beef broth, ensuring there are no lumps. Stir in the ketchup, mustard, and the remaining 1 tablespoon of Worcestershire sauce. Bring the mixture to a simmer, stirring occasionally.
Step 9: Return the seared Salisbury steak patties to the skillet, nestling them into the gravy. Reduce the heat to low, cover the skillet, and simmer for 15-20 minutes, or until the patties are cooked through and tender, and the gravy has thickened to your desired consistency.
Step 10: Taste the gravy and adjust seasonings (salt and pepper) as needed. Garnish with fresh chopped parsley, if desired. Serve hot with creamy mashed potatoes, egg noodles, or rice.