In a large bowl, combine ground beef, breadcrumbs, milk, onion, egg, Worcestershire sauce, garlic powder, salt, and black pepper. Mix until well combined.

Shape the mixture into 4 oval patties.

In a large skillet, heat olive oil 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 mushrooms and sauté for about 5 minutes until they are tender.

Pour in beef broth and soy sauce, bringing the mixture to a simmer.

Return the patties to the skillet and cover. Cook for an additional 10 minutes, or until the patties are cooked through.

Stir in the cornstarch mixture and cook for another 2-3 minutes until the gravy thickens.

Serve the Salisbury steaks topped with mushroom gravy and garnish with chopped parsley.