In a large bowl, combine ground beef, breadcrumbs, milk, egg, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Mix until well combined. Form 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, or until browned and cooked through. Remove the patties from the skillet and set aside.
In the same skillet, add sliced onions and mushrooms. Sauté for about 5-7 minutes until the onions are translucent and the mushrooms are tender.
Pour in the beef broth and bring to a simmer. Return the patties to the skillet and let them cook in the gravy for about 10 minutes.
In a small bowl, mix cornstarch and water to create a slurry. Stir the slurry into the gravy and cook for an additional 2-3 minutes until the gravy thickens.
Serve the Salisbury steaks topped with mushroom gravy and garnish with fresh parsley.