To prepare sauce, drain chicken, reserving liquid. Add enough milk to reserved liquid to measure 2 C, set aside. Combine butter and flour, cook over medium heat 1 minute. Stir in milk mixture gradually, whisking constantly over medium heat. Season to taste with salt, pepper and nutmeg. Mix 1/2 sauce with spaghetti and mushrooms in a lightly greased 2 qt casserole. Stir in chicken and remaining sauce. Pour over chicken mixture. Sprinkle with cheese and crumbs.
Bake for 30 minutes.
