Step 1: In a large bowl, combine the ground beef, breadcrumbs, egg, half of the minced onion (about 2 tablespoons), 1 tablespoon of Worcestershire sauce, 1/2 teaspoon of Dijon mustard, garlic powder, salt, and black pepper. Gently mix with your hands until just combined, being careful not to overmix.
Step 2: Divide the mixture into 4-6 equal portions and form them into oval-shaped patties, about ¾ inch thick. Create a slight indentation in the center of each patty for even cooking.
Step 3: Heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Carefully place the patties in the hot skillet (work in batches if necessary) and sear for 3-4 minutes per side, until nicely browned. Remove the seared patties to a plate and set aside.
Step 4: Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil (if needed) and the sliced mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms are softened, browned, and have released their liquid, about 5-7 minutes.
Step 5: Stir in the remaining minced onion (about 2 tablespoons) and cook for another 2-3 minutes until softened. Add the minced garlic and cook for 1 minute more until fragrant. Sprinkle the flour over the vegetables and cook, stirring constantly, for 1-2 minutes to create a roux.
Step 6: Gradually whisk in the beef broth, scraping up any browned bits from the bottom of the pan. Stir in the remaining 1 tablespoon of Worcestershire sauce, 1/2 teaspoon of Dijon mustard, and the sprig of fresh thyme or rosemary (if using). Bring the gravy to a simmer, stirring until it begins to thicken.
Step 7: Return the seared salisbury steak patties to the skillet, nestling them into the gravy. Reduce the heat to low, cover, and simmer for 10-15 minutes, or until the patties are cooked through and the gravy has reached your desired consistency. The internal temperature of the patties should reach 160°F (71°C).
Step 8: Remove and discard the thyme/rosemary sprig before serving. Taste the gravy and adjust seasoning with salt and pepper as needed. Serve your classic salisbury steak recipe hot, garnished with fresh chopped parsley, alongside mashed potatoes, egg noodles, or rice.