Add beef, breadcrumbs, egg, mustard, ketchup and worcestershire sauce to a large bowl. Combine with your hands or a large spoon. Roll into 1 1/2 inch meatballs.
Heat oil in a large skillet. Working in batches add meatballs and brown on both sides, 2 minutes per side. (They will finish cooking in the sauce). Remove the meatballs and set aside.
Add onions to the same skillet and cook over medium heat until light brown and caramelized 5-10 minutes. (You can add more oil or a splash of water if the onions are getting too brown)
Add the butter to the skillet with the onions and melt.Next add in the garlic and sauté for 1-2 minutes.
Add the flour to the skillet and whisk until it forms a thick paste. Cook for 1 minute whisking constantly.
Slowly pour in the beef broth until it forms a thick gravy.Add in worcestershire and season with salt and pepper to taste.
Return the meatballs back to the pan and simmer for 3-4 minutes or until the meatballs are cooked through.
Serve immediately over egg noodles or mashed potatoes with fresh parsley for garnish.