Cook noodles according to package directions, omitting salt and fat; drain.
While noodles cook, heat a large Dutch oven over high heat; generously coat pan with cooking spray. Add mushrooms and onion; cook 10 minutes or until browned, stirring frequently.
While mushroom mixture cooks, place flour in a bowl. Gradually add milk, stirring with a whisk until smooth. Add sherry and next 3 ingredients, stirring with a whisk. Transfer cooked mushroom mixture to a large bowl. Gradually add milk mixture to hot pan, stirring with a whisk. Cook, whisking constantly, 3 minutes or until slightly thickened.
Stir sauce and cooked noodles into mushroom mixture. Stir in sour cream; sprinkle with chives, if desired. Serve immediately.
Shortcut kitchen tip
To chop chives quickly and safely, hold the chives in a bunch, and place them on a clean cutting board. With a sharp knife, chop the chives to the desired size. To prepare the chives ahead, chop them, and place in a zip-top plastic bag; store in the refrigerator until you need them.
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