Cook spaghetti according to package directions. Meanwhile, in a Dutch oven, melt butter. Stir in the flour, salt and pepper until smooth. Gradually add the broth, half-and-half and whipping cream. Bring to a boil; cook and stir until thickened, about 2 minutes. , Remove from the heat. Stir in the chicken, mushrooms and pimientos. Drain spaghetti; add to the chicken mixture and toss to coat., Transfer to 2 greased 11x7-in. baking dishes. Sprinkle with cheese. Cover and freeze 1 casserole for up to 2 months. Bake the second casserole, uncovered, at 350° until heated through, 20-25 minutes. , To use frozen casserole: Thaw in the refrigerator overnight. Cover and bake at 350° for 30 minutes. Uncover; bake until heated through, 15-20 minutes more. Stir before serving.