Preheat oven to 350°. Cook spaghetti according to package directions. , Meanwhile, in a Dutch oven, saute onions and peppers in butter until tender. Stir in flour until blended; gradually add milk. Bring to a boil; cook and stir 2 minutes or until thickened. Stir in chicken, soups, sour cream, celery salt and pepper., Drain spaghetti; add to sauce mixture and toss to coat. Transfer to 2 greased 11x7-in. baking dishes. Sprinkle with cheeses. Cover and bake 20 minutes. Uncover and bake 5-10 minutes longer or until bubbly. Freeze option: Cover and freeze unbaked casseroles up to 3 months. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Preheat oven to 350°. Cover casserole with foil; bake as directed, increasing covered time to 40 minutes or until heated through and a thermometer inserted in center reads 165°. Uncover; bake 5-10 minutes longer or until bubbly.