In a large bowl, combine the ground beef, breadcrumbs, chopped onion, egg, Worcestershire sauce, garlic powder, salt, and black pepper. Mix until well combined, then shape 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 the sliced mushrooms and sauté for about 5 minutes, or until they are tender and browned.

Pour in the beef broth and soy sauce, stirring to combine. Bring the mixture to a simmer.

In a small bowl, mix the cornstarch and water until smooth. Slowly add this mixture to the skillet, stirring constantly until the sauce thickens.

Return the cooked patties to the skillet, spooning the mushroom sauce over them. Simmer for an additional 5 minutes to heat through.

Serve hot, garnished with chopped parsley.