In a large bowl, mix ground beef, breadcrumbs, egg, chopped onion, garlic, Worcestershire sauce, salt, and pepper. Form into medium-sized meatballs.Heat a skillet over medium heat and cook the meatballs until browned on all sides. Remove and set aside.In the same pan, melt butter and add flour. Cook for 1–2 minutes while stirring continuously.Slowly add beef broth while stirring to avoid lumps. Add ketchup, mustard, and garlic powder. Simmer until thick gravy forms.Add meatballs back into the gravy and simmer for 10 minutes.Boil potatoes in salted water until soft. Drain and mash until smooth.In a small pan, sauté garlic in butter until fragrant. Add to mashed potatoes along with milk and salt. Mix until creamy.Stir in chopped parsley and serve hot with meatballs and gravy.