In a skillet, cook turkey and onion until meat is no longer pink; drain. Add mushrooms; cook and stir for 1 minute. Stir in the soup, buttermilk, garlic powder, salt and pepper. Bring to a boil; reduce heat. Simmer, uncovered, for 5-10 minutes. Stir in sour cream; heat gently but do not boil. Serve over noodles. Garnish with parsley if desired.