Cook zucchini and onion in salted water for 5 minutes. Drain. Combine mushroom soup, sour cream and carrots and mix into zucchini and onion mixture. In separate bowl, combine melted butter and stove-top stuffing.
Use half of stuffing to line greased casserole dish. Put zucchini-carrot mixture on top of stuffing. Top with remaining half of stuffing.