Step 1Heat the oven to 350 degrees.
Step 2Cook the zucchini in boiling salted water until crisp-tender, about 10 minutes. Drain.
Step 3Melt 4 tablespoons of butter in a saucepan and cook the onion until tender, about 5 minutes. Stir in the shredded carrots and cook a few minutes more.
Step 4Remove the pan from the heat and add 1 1/2 cups of stuffing, the soup and the sour cream. Gently stir in the zucchini. Turn into a 1 1/2-quart casserole.
Step 5Melt the remaining 2 tablespoons of butter in a sauce pan and add the remaining stuffing. Sprinkle the bread cubes on top of the casserole. Bake until heated through, 30 to 40 minutes.