Step 1: In a large bowl, combine ground beef, breadcrumbs, 1/4 cup finely chopped onion, egg, 1 tbsp Worcestershire sauce, garlic powder, salt, and pepper. Mix gently until just combined, being careful not to overmix.
Step 2: Divide the mixture into 4 equal portions and shape each into an oval-shaped patty, about 3/4-inch thick.
Step 3: Heat olive oil in a large skillet over medium-high heat. Carefully place the patties in the hot skillet and cook for 4-5 minutes per side, until browned. Remove patties from the skillet and set aside. Do not drain the drippings.
Step 4: In the same skillet, melt butter over medium heat. Add sliced mushrooms and 1/2 cup finely chopped onion. Cook, stirring occasionally, until mushrooms are softened and onions are translucent, about 5-7 minutes.
Step 5: Add minced garlic and cook for 1 minute more until fragrant.
Step 6: Sprinkle flour over the mushrooms and onions, stirring constantly for 1-2 minutes to create a roux.
Step 7: Gradually whisk in beef broth and 1 tbsp Worcestershire sauce, scraping up any browned bits from the bottom of the skillet. Bring to a simmer, stirring until the gravy thickens.
Step 8: Stir in Dijon mustard. Season the gravy with salt and black pepper to taste.
Step 9: Return the browned Salisbury steak patties to the skillet, nestling them into the gravy. Reduce heat to low, cover, and simmer for 10-15 minutes, or until the patties are cooked through and the gravy is rich and flavorful.
Step 10: Serve hot, garnished with fresh parsley if desired.