Step 1: In a large bowl, gently combine the ground beef, 1/4 cup of the finely diced onion, breadcrumbs, egg, Dijon mustard, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix until just combined – do not overmix. Form the mixture into 4-6 oval-shaped patties, about 1/2 to 3/4 inch thick. Make a slight indentation in the center of each patty with your thumb.
Step 2: Heat olive oil in a large skillet over medium-high heat. Carefully place the patties in the hot skillet. Brown them for 3-4 minutes per side until a deep crust forms. Remove the browned patties and set them aside on a plate.
Step 3: Reduce the heat to medium. Add the remaining finely diced onion and sliced mushrooms to the skillet. Cook, stirring occasionally, for 5-7 minutes until the onions are softened and translucent, and the mushrooms have released their moisture and started to brown.
Step 4: Sprinkle the flour over the cooked onions and mushrooms. Stir continuously for 1-2 minutes to cook out the raw flour taste. Gradually whisk in the beef broth and Worcestershire sauce until smooth. Bring the gravy to a simmer, stirring, until it thickens to your desired consistency. Season with the remaining 1/2 teaspoon salt and 1/4 teaspoon black pepper to taste.
Step 5: Return the browned Salisbury steak patties to the skillet, nestling them into the simmering gravy. Cover the skillet, reduce the heat to low, and let it simmer for 15-20 minutes, or until the patties are cooked through (internal temperature 160°F/71°C) and tender.
Step 6: Garnish with fresh chopped parsley, if desired. Serve the Easy Salisbury Steak Recipe immediately with your favorite side dishes.