1. In a bowl, mix ground beef, breadcrumbs, egg, diced onion, Worcestershire sauce, garlic powder, salt, and pepper. Form into 4 patties.
2. Heat a skillet over medium-high heat and cook the patties until browned on both sides. Remove and set aside.
3. In the same skillet, melt butter and sauté mushrooms and sliced onions until softened. Stir in flour and cook for 1 minute.
4. Gradually add beef broth, tomato paste, and Worcestershire sauce, whisking to combine. Simmer until thickened.
5. Return the patties to the skillet and simmer in the gravy for 10 minutes. Adjust seasoning as needed.
6. Serve hot over mashed potatoes, garnished with fresh parsley.