Cook spaghetti according to package directions. Meanwhile, in a large bowl, combine the chicken, soup, onion, milk, 1/2 cup cheddar cheese, 1/2 cup Swiss cheese, mushrooms, peppers, mayonnaise, steak seasoning and basil., Drain spaghetti. Add to chicken mixture; toss to coat. Transfer to a greased 13x9-in. baking dish. Cover and bake at 350° for 20 minutes. Uncover; sprinkle with remaining cheeses. Bake until heated through and cheese is melted, 5-10 minutes longer., Freeze option: Cover and freeze unbaked casserole. To use, partially thaw in refrigerator overnight. Remove from refrigerator 30 minutes before baking. Bake as directed, increasing time as needed to heat through.