Cook the pasta as directed on the package. Set aside to keep warm. Meanwhile, in a large nonstick skillet, cook the onion in the butter over medium-high heat for 2 minutes, stirring occasionally. Stir in the mushrooms and cook for 3 minutes, stirring occasionally. Stir in the meatballs and 1 1/4 cups of the broth. Bring to a boil. Cover and simmer for 12 to 18 minutes or until the meatballs are heated through and the internal temperature is 165 degrees. In a small bowl, mix the remaining 1/4 cup broth and flour. Whisk until smooth. Stir into the meatball mixture. Cook for 1 minute. Remove from the heat and stir in the sour cream. Serve the meatballs and sauce over the noodles. Sprinkle with parsley before serving.