In a large mixing bowl, combine the ground beef, egg, chopped onion, seasoned breadcrumbs, Worcestershire sauce, and salt and pepper. Mix gently with your hands until all ingredients are fully incorporated.

Divide the mixture into four even portions and shape each into a patty.

Heat the olive oil in a large skillet over medium-high heat. Add the patties and sear on both sides until browned, about 3–4 minutes per side. Remove the patties from the skillet and set aside.

Pour out any excess fat from the skillet, leaving a small amount for flavor. Add the butter and allow it to melt.

Add the sliced mushrooms to the skillet and sauté until they begin to soften, about 3–5 minutes.

Sprinkle the flour over the mushrooms and stir well to coat evenly. Cook for 1–2 minutes to remove the raw flour taste.

Gradually pour in the beef broth while stirring continuously, making sure the flour is fully incorporated and smooth.

Return the seared patties to the skillet. Reduce the heat to medium-low and simmer for 10–15 minutes, or until the patties are cooked through and the gravy has thickened. Taste and adjust seasoning with salt and pepper as needed.

Serve the Salisbury steaks hot, topped with the mushroom gravy, alongside creamy mashed potatoes or your favorite side vegetables.