In a large bowl, combine ground beef, bread crumbs, milk, egg, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Mix until well combined. Shape the mixture into 4 oval patties.
Heat olive oil in a large skillet over medium heat. Add the patties and cook for about 4-5 minutes on each side until browned. Remove the patties from the skillet and set aside.
In the same skillet, add sliced onion and mushrooms. Sauté for about 5 minutes until softened. Pour in the beef broth and bring to a simmer.
In a small bowl, mix cornstarch and water to create a slurry. Stir the slurry into the skillet along with soy sauce. Cook for an additional 2-3 minutes until the gravy thickens.
Return the patties to the skillet, spoon the gravy over them, and simmer for another 5 minutes until heated through.
Serve the Salisbury steak hot, garnished with fresh parsley.