1. Steam zucchini and onion until barely soft; drain.
2. Put 1/2 stick of butter in bottom of casserole (cut into pieces). Mix soup, sour cream and carrots well. Fold in zucchini and onion and pour mixture into casserole dish. Melt 1/3 cube butter and mix with crumbs, put over top of casserole.
3. Bake uncovered at 350 degrees F for 35-40 minutes.