Boil zucchini and onion in salted water 10 minutes or until done. Drain and set aside. Toss stuffing crumbs and seasoning packet with melted butter or margarine. Put 1/2 in bottom of 9x13-inch baking pan. Mix together sour cream, cream of chicken soup and cheddar cheese. Add zucchini and mix well. Spoon over stuffing mixture in pan. Top with remaining stuffing mixture. Bake at 350F for 25 minutes.