Mix the ground beef, panko, egg, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Form into one-inch meatballs.
Sear the meatballs in olive oil over medium-high heat until browned on all sides, then remove to a plate.
Cook the mushrooms and onions in the same skillet with butter until softened. Stir in the flour and cook for one minute.
Whisk in the beef broth, scraping up browned bits. Simmer until thickened, return meatballs to the pan, and cook for 10 minutes.
Boil the potatoes until tender. Drain and mash with warm heavy cream, softened butter, minced garlic, and fresh parsley. Serve the meatballs and gravy over the potatoes.