Combine zucchini and onion in medium kettle, add water to cover and bring to a boil. Boil for 5 minutes; drain and cool. Combine carrots, soup, sour cream and garlic powder in a large bowl. Add zucchini/onion and chicken; mix.
Spread in a greased 13 x 9 inch baking dish. For topping, melt butter in a skillet, add bread stuffing and seasoning packet; toss well. Sprinkle over casserole. Top with cheese if desired. Bake at 350 degrees for 1 hour or until golden brown.