Step 1: In a large bowl, combine the ground beef, bread crumbs, ketchup, Worcestershire sauce, mustard powder, egg, salt, and pepper. Mix gently with your hands until just combined. Form the mixture into four oval-shaped patties, about 1-inch thick.
Step 2: Heat the olive oil and 1 tablespoon of butter in a large, oven-safe skillet over medium-high heat. Add the patties and sear for 3-4 minutes per side, until nicely browned. The patties don't need to be cooked through at this stage. Remove the patties from the skillet and set aside on a plate.
Step 3: Reduce the heat to medium. Add the remaining 1 tablespoon of butter to the skillet. Add the sliced onion and mushrooms (if using) and cook, stirring occasionally, for 5-7 minutes until the onions are softened and lightly caramelized, and the mushrooms have released their moisture and browned.
Step 4: Sprinkle the all-purpose flour over the cooked onions and mushrooms. Stir continuously for 1-2 minutes, allowing the flour to cook and form a light brown roux.
Step 5: Gradually pour in the beef broth, whisking constantly to prevent lumps. Bring the gravy to a simmer, scraping up any browned bits from the bottom of the pan to incorporate them into the gravy. Continue to whisk until the gravy begins to thicken, about 3-5 minutes.
Step 6: Carefully place the seared Salisbury steak patties back 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 gravy has reached your desired consistency. Season the gravy with additional salt and pepper to taste.
Step 7: Remove from heat and serve hot. Garnish with fresh chopped parsley, if using, and serve immediately with your favorite sides like mashed potatoes or egg noodles.