Step 1: In a large bowl, gently combine the ground beef, panko breadcrumbs, milk, egg, 1 tbsp Worcestershire sauce, minced garlic, onion powder, 1/2 tsp salt, and 1/4 tsp black pepper. Mix just until combined; do not overmix, as this can make the patties tough.
Step 2: Divide the mixture into four equal portions and form them into oval-shaped patties, about 3/4-inch thick. Make 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 in a large skillet or Dutch oven over medium-high heat. Once hot, sear the patties for 3-4 minutes per side, until deeply browned. They don't need to be cooked through at this stage. Remove the patties from the skillet and set aside on a plate, reserving the drippings in the pan.
Step 4: Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil to the skillet with the beef drippings. Add the sliced onions and cook, stirring occasionally, until softened and lightly caramelized, about 5-7 minutes.
Step 5: Add the sliced cremini mushrooms to the skillet. Cook, stirring occasionally, until the mushrooms have released their moisture and started to brown, about 7-10 minutes.
Step 6: Sprinkle the flour over the onions and mushrooms. Cook, stirring constantly, for 1-2 minutes to toast the flour, creating a roux.
Step 7: Gradually whisk in the beef broth, scraping up any browned bits from the bottom of the skillet. Stir in the extra 1 tbsp Worcestershire sauce and Dijon mustard. Bring the gravy to a simmer, stirring until it begins to thicken. Season with salt and pepper to taste.
Step 8: Return the seared Salisbury steak patties to the skillet, nestling them into the simmering mushroom gravy. Reduce the heat to low, cover, and let simmer for 15-20 minutes, or until the patties are cooked through and tender (internal temperature 160°F / 71°C).
Step 9: Taste the gravy and adjust seasonings if necessary. If the gravy is too thick, add a splash more beef broth. If too thin, you can simmer uncovered for a few more minutes or whisk in a cornstarch slurry (1 tsp cornstarch mixed with 1 tbsp cold water) until desired thickness.
Step 10: Serve the Salisbury steak hot, garnished with fresh chopped parsley if desired, typically over mashed potatoes or egg noodles.