1. Sauté onion in a lightly greased large saucepan over medium heat 2 minutes. Add mushrooms, and sauté 2 to 3 minutes or until onion is tender.
2. Whisk together beef broth and next 3 ingredients in a medium bowl until smooth. Add soup mixture and meatballs to onion mixture in pan. Cook meatballs according to time on package directions, stirring occasionally.
3. Meanwhile, prepare noodles according to package directions.
4. Remove meatball mixture from heat; stir in sour cream until smooth and blended. Serve meatball mixture over hot cooked noodles. Garnish, if desired.