1Cook noodles as directed on package, omitting salt and oil; drain.
2Generously spray 5- to 6-quart Dutch oven with butter-flavor cooking spray; heat over high heat. Add mushrooms and onion; cook 10 minutes, stirring frequently, until vegetables are golden brown.
3Meanwhile, place flour in medium bowl. Gradually beat in milk with wire whisk until smooth. Beat in sherry, butter, salt and pepper. Pour cooked mushroom mixture into large bowl. Gradually add milk mixture to hot Dutch oven, stirring with wire whisk. Cook 3 minutes, stirring constantly with whisk, until slightly thickened.
4Stir sauce and noodles into mushroom mixture. Stir in sour cream. Sprinkle with chives.