In a large bowl, combine the ground beef, bread crumbs, chopped onion, egg, Worcestershire sauce, salt, and pepper. Mix well until all ingredients are thoroughly combined.

Divide the beef mixture into equal portions and shape them into oval patties, about 1 inch thick.

In a large skillet, heat the cooking oil over medium heat. Once hot, add the patties and sear them for about 4-5 minutes on each side or until a golden crust forms. Remove the patties from the skillet and set aside.

In the same skillet, add the sliced mushrooms and sauté until tender. Sprinkle the flour over the mushrooms and stir for about a minute to create a roux.

Gradually add the beef broth, stirring constantly to avoid lumps. Bring the mixture to a simmer and let it thicken for about 2-3 minutes.

Return the patties to the skillet, spoon some gravy over them, and cover. Let everything simmer on low heat for an additional 10 minutes to ensure the flavors meld beautifully.

Plate your Salisbury Steaks, generously ladling the mushroom gravy over the top, and serve hot with mashed potatoes or rice.