In a large saucepan, bring the broth, water and onion to a boil. Reduce heat. Add rice, cover and simmer for 15 minutes. Remove from the heat; fluff with a fork.
Divide rice between two greased 9-inch square baking pans. Sprinkle each with peas and turkey. In a bowl, combine the soup, milk and salt until smooth; pour over turkey. Toss the cracker crumbs and butter, sprinkle over the top.
Cover and freeze one casserole for up to 3 months. Bake the second casserole, uncovered, at 350 degrees for 35 minutes or until golden brown.
To use frozen casserole: Thaw in the refrigerator for 24 hours. Bake, uncovered, at 350 degrees for 45-50 minutes or until heated through.