In a large bowl, combine the ground beef, bread crumbs, egg, onion, salt, black pepper, and Worcestershire sauce. Mix until well combined.
Form the mixture into 4 oval patties.
In a skillet over medium-high heat, brown the patties on both sides. Remove and set aside.
In the same skillet, add the mushrooms and cook until softened.
Add the beef broth and bring to a simmer. Return the patties to the skillet.
Cover and cook for 20 minutes, or until the patties are cooked through.
Remove the patties again, and stir the cornstarch slurry into the skillet. Cook until the sauce thickens.
Serve the Salisbury Steaks topped with the mushroom gravy.