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

Shape the mixture into 4 oval patties, about 1/2 inch thick.

In a large skillet, heat the olive oil over medium heat. Add the patties and cook for about 4-5 minutes on each side, or until browned and cooked through. Remove the patties from the skillet and set aside.

In the same skillet, add the sliced mushrooms and sauté for about 5 minutes, or until they are tender.

Pour in the beef broth and bring to a simmer.

In a small bowl, mix the cornstarch and water to create a slurry. Gradually add the slurry to the simmering broth, stirring constantly until the gravy thickens.

Return the patties to the skillet, spoon the gravy over them, and let them simmer for an additional 5 minutes.

Serve the Salisbury steak hot, garnished with fresh parsley.