Step 1: In a large bowl, combine the ground beef, breadcrumbs, egg, finely chopped onion, 1/2 tablespoon Worcestershire sauce, Dijon mustard, garlic powder, salt, and black pepper. Mix gently with your hands until just combined, being careful not to overmix.
Step 2: Divide the mixture into 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 to prevent bulging during cooking.
Step 3: Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the patties to the skillet, working in batches if necessary, and sear for 3-4 minutes per side until deeply browned and a crust has formed.
Step 4: Remove the browned patties from the skillet and set them aside on a plate.
Step 5: 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 they are softened and have released their liquid. Scrape up any browned bits from the bottom of the pan.
Step 6: Sprinkle the flour over the mushrooms and cook for 1-2 minutes, stirring constantly, until the flour is absorbed and lightly toasted.
Step 7: Gradually whisk in the beef broth and the remaining 1/2 tablespoon of Worcestershire sauce, ensuring no lumps form. Bring the mixture to a simmer, stirring occasionally, until the gravy begins to thicken.
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 (internal temperature of 160°F / 71°C).
Step 9: Remove from heat and let rest for a few minutes before serving. Garnish with fresh chopped parsley if desired. Serve hot with mashed potatoes, egg noodles, or rice.