Boil the zucchini and onion in water for 5 minutes; drain well. In a medium bowl, combine the soup, sour cream and carrots. Stir in the zucchini and onion and mix well.
In a separate medium bowl, combine the butter and stuffing mix. Spread half of this mixture into the bottom of a 9x13-inch baking dish. Spoon the zucchini mixture over the stuffing, then top off with the other half of the stuffing.
Bake at 350 degrees F (175 degrees C) for 25 to 30 minutes, or until stuffing is golden brown.