In a large mixing bowl, combine the ground beef, breadcrumbs, ketchup, mustard, Worcestershire sauce, dried parsley, garlic powder, salt, black pepper, and egg. Mix the ingredients thoroughly until well combined.
Shape the beef mixture into individual oval-shaped patties, approximately 1 inch thick.
Heat the olive oil in a skillet or frying pan over medium-high heat. Add the patties and cook for about 4-5 minutes on each side, or until they are browned and cooked through. Once cooked, transfer the patties to a plate and cover them with foil to keep them warm.
In the same skillet, melt the butter and add the sliced mushrooms, chopped onion, and minced garlic. Cook for approximately 5 minutes, or until the vegetables are soft and lightly browned.
In a small bowl, whisk together the cornstarch and water until smooth, creating a slurry. Set it aside.
Add the beef broth, Worcestershire sauce, ketchup, soy sauce, salt, and black pepper to the skillet with the mushrooms and onions. Thoroughly stir and gently bring the mixture to a simmer.
Gradually whisk in the cornstarch slurry into the skillet, stirring constantly. Continue cooking and stirring until the gravy thickens, usually for about 2-3 minutes.
Return the cooked beef patties to the skillet, nestling them into the mushroom gravy. Allow them to simmer for an additional 5 minutes, ensuring the patties are coated in the gravy and heated through.
Once ready, remove the skillet from the heat. Serve the Pioneer Woman Salisbury Steak hot, spooning the mushroom gravy over each patty.