In a large skillet, combine meatballs, broth, and mushrooms. Bring to boiling; reduce heat. Cover and simmer about 15 minutes or until meatballs are heated through.
In a small bowl, stir together sour cream and flour. Whisk in milk and mustard. Stir sour cream mixture into skillet. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Serve over hot cooked noodles. If desired, stir in snipped fresh parsley. Makes 6 to 8 servings.