Break spaghetti into pieces. Cook according to package directions. Drain. Transfer to a very large mixing bowl.
In a large skillet, cook mushrooms and onion in butter until vegetables are tender, but not brown. Remove from heat; add chicken. Set aside.
Stir together cream of chicken soup and sour cream and add to drained spaghetti. Stir in chicken mixture and water chestnuts. Spoon into a 13x9x2-inch (3-quart rectangular) baking dish. Sprinkle with Parmesan cheese.
Cover and bake in a 350 degree F oven for 30 minutes. Uncover and bake 15 minutes more or until edges are bubbly and mixture is heated through. Sprinkle parsley on top. Makes 8 to 10 servings.