1. Cook spaghetti according to package directions. Meanwhile, in a Dutch oven, saute the onions and peppers in butter until tender. Stir in flour until blended; gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in the chicken, soups, sour cream, celery salt and pepper.
2. Drain spaghetti; add to sauce mixture and toss to coat. Transfer to two greased 11-in. x 7-in. baking dishes. Sprinkle with cheeses. Cover and freeze one casserole for up to 3 months. Cover and bake the remaining casserole at 350° for 20 minutes. Uncover and bake 5-10 minutes longer or until bubbly.
3. To use frozen casserole: Thaw in the refrigerator overnight. Remove from the refrigerator 30 minutes before baking. Cover and bake at 350° for 40 minutes. Uncover and bake 5-10 minutes longer or until bubbly. Yield: 2 casseroles (6 servings each).