Mix the Meat: In a large mixing bowl, combine the ground beef, egg, seasoned breadcrumbs, Worcestershire sauce, chopped onion, salt, and pepper. Gently knead with your hands until the ingredients are just incorporated. Do not overmix.

Shape the Patties: Divide the mixture into four even portions. Form them into oval-shaped patties, about 3/4-inch thick. Press a slight indentation into the center of each patty.

Sear the Patties: Heat the olive oil in a large skillet over medium-high heat. Add the patties and sear until a rich brown crust forms on both sides, about 3-4 minutes per side. Remove the patties from the skillet and set aside. Carefully pour off any excess heavy grease, keeping the browned bits in the pan.

Sauté the Mushrooms: Reduce the heat to medium. Add the butter to the skillet. Once melted, add the sliced mushrooms. Sauté until they are softened and lightly browned, about 5-7 minutes.

Create the Gravy: Sprinkle the all-purpose flour over the mushrooms and stir continuously for 1 to 2 minutes to cook off the raw flour taste.

Add the Liquid: Slowly pour in the beef broth, whisking continuously to ensure the flour dissolves smoothly and no lumps form. Bring the gravy to a gentle simmer.

Simmer and Serve: Return the seared patties (and any accumulated juices) to the skillet. Reduce the heat to medium-low, and let the steaks simmer in the gravy for 10-15 minutes, until the meat is fully cooked and the gravy has thickened to your liking.

Enjoy: Prepare the mashed potatoes according to the package directions. Serve the Salisbury steak hot over a bed of mashed potatoes, generously ladled with the mushroom gravy.