Preheat oven to 375°F (190°C). Spray a large baking dish (like 9x13 inch) with cooking spray.
Prep your veggies: slice zucchini, chop onion, shred carrots. If zucchini is very wet, pat it dry.
In a large bowl, combine sliced zucchini, chopped onion, shredded carrots, sour cream, and cream of chicken soup. Stir gently until coated. Add salt and pepper if desired.
In a separate medium bowl, combine the dry stuffing mix with the softened butter. Mix until crumbly using a fork or fingers.
Pour the vegetable mixture evenly into the prepared baking dish.
Sprinkle the buttery stuffing mixture evenly over the top of the vegetables.
Bake for 30-40 minutes, or until the casserole is bubbly around the edges and the stuffing topping is golden brown and crispy.
Let rest for a few minutes before serving.