In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Mix until just combined, then shape 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 onion and mushrooms. Sauté for about 5-7 minutes until softened.
Pour in the beef broth and bring to a simmer.
Return the patties to the skillet, cover, and let simmer for about 10 minutes until cooked through.
In a small bowl, mix cornstarch and water to create a slurry. Stir this into the gravy and cook for an additional 2-3 minutes until thickened.
Serve the Salisbury steak topped with mushroom gravy and garnish with chopped parsley.