In a large bowl, combine ground beef, bread crumbs, milk, egg, chopped onion, salt, and pepper. Mix gently until just combined.

Divide the mixture into 4 equal portions and shape them into oval patties.

Heat 1 tablespoon of oil in a large skillet over medium-high heat. Sear the patties for 2-3 minutes per side until browned.

Remove the patties from the skillet and set aside.

In the same skillet, melt the butter over medium heat. Add the sliced mushrooms and chopped onion. Cook until softened and lightly browned, about 5-7 minutes.

Sprinkle the flour over the mushrooms and onions and stir for 1 minute.

Gradually whisk in the beef broth and Worcestershire sauce until smooth.

Bring to a simmer and cook, stirring occasionally, until the gravy has thickened, about 5-7 minutes.

Season the gravy with salt and pepper to taste.

Return the Salisbury steak patties to the skillet with the gravy. Spoon some gravy over the patties.

Cover and simmer for 10-15 minutes, or until the patties are cooked through.

Serve hot, spooning extra gravy over the top.