Cook squash and onion in boiling salted water for 5 minutes; drain.
Combine sour cream and soup in a bowl . Stir in carrots. Fold in zucchini.
Mix stuffing with melted butter. Place half the stuffing in the bottom of a 2 quart casserole. Cover with zucchini. Top with remaining stuffing. (Can be covered and refrigerated at this point.)
Bring to room temperature. Preheat oven to 350F and bake 25-30 minutes