Step 1: In a large bowl, combine the ground beef, breadcrumbs, 1/4 cup chopped onion, egg, 2 tablespoons Worcestershire sauce, garlic powder, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix gently with your hands until just combined, being careful not to overmix.
Step 2: Divide the mixture into 6-8 equal portions and shape them into oval-shaped patties, about 3/4-inch thick.
Step 3: Heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Once hot, add the patties in a single layer, working in batches if necessary to avoid overcrowding. Sear for 3-4 minutes per side until a deep brown crust forms.
Step 4: Remove the seared patties from the skillet and set them aside on a plate. Reduce the heat to medium.
Step 5: If needed, add the remaining 1 tablespoon of olive oil to the same skillet. Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until they are softened and lightly browned.
Step 6: Stir in the 1/2 cup chopped onion and minced garlic. Cook for another 2-3 minutes until the onion is softened and fragrant.
Step 7: Sprinkle the all-purpose flour over the vegetables and stir constantly for 1 minute to cook the flour. This creates a roux that will thicken the gravy.
Step 8: Gradually whisk in the beef broth, 1 tablespoon Worcestershire sauce, and Dijon mustard. Bring the gravy to a gentle simmer, stirring occasionally, until it thickens to your desired consistency.
Step 9: Season the gravy with salt and black pepper to taste. Adjust as needed.
Step 10: Carefully return the seared Salisbury steak patties to the skillet, nestling them into the mushroom gravy. Reduce the heat to low, cover, and simmer for 15-20 minutes, or until the patties are cooked through and tender.
Step 11: Garnish with fresh chopped parsley, if desired, and serve immediately with your favorite sides.