Add all ingredients for the meatballs EXCEPT for the butter, flour, broth, 1 cup of milk and mustard to a large bowl. Knead very well. Using about 2 teaspoons of meat mixture, roll into firm balls. Place in a baking sheet.
Bake the meatballs, place them on a lined baking sheet and bake them at 400°F for 10-12 minutes or until browned and no longer pink in the middle.
Bring the skillet back to medium heat. Melt the butter, then stir in the flour and cook, whisking constantly, until medium brown. Pour in the beef broth while whisking constantly. Simmer until thickened, then whisk in the milk and mustard. Simmer gently over medium-low heat until creamy, about 1 minute. Season to taste.
Place the meatballs back in the gravy and warm very gently until heated through. I like to serve them over mashed potatoes or pappardelle pasta.Enjoy!