Combine squash and water in a saucepan and simmer, covered, 10 minutes or until tender. Mash in a colander and drain well.
Saute onions in butter. Combine onions, squash, 3/4 cup cracker crumbs, cheese, remaining ingredients. Spoon into greased 2 quart casserole. Sprinkle with remaining 1/4 cup cracker crumbs. Bake 350 degrees for 45 minutes or until cooked through. 6 servings.