Cook pasta according to package directions. Meanwhile, in a Dutch oven, saute the celery, pepper and onion in butter until tender. Stir in the soup, milk and salt. In a very large bowl, combine the soup mixture, ham and mushrooms. Drain pasta; add to soup mixture and toss to coat.
Transfer to two greased 13x9-in. baking dishes. Top with cracker crumbs. Cover and freeze one casserole for up to 3 months. Bake the remaining casserole, uncovered, at 350° for 20-25 minutes or until golden brown.
To use frozen casserole: Thaw in the refrigerator overnight. Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 350° for 50-55 minutes or until heated through.