Place squash and onion in pan, and add water to cover. Bring to boil, cover, and cook 15 minutes, or until tender. Drain well and lightly mash.
Combine squash mixture, sour cream, and next 3 ingredients, and mix well.
Ladle into greased 2-quart casserole.
Preheat oven to 300 degrees.
Combine cracker crumbs and remaining 3 ingredients; toss well. Sprinkle over squash.
Bake uncovered for 30 minutes or until crust is firm and dish is thoroughly heated.
Serves 6-8.