To a large skillet over medium heat, add the butter. Once it melts down, whisk in the flour. Once the flour turns a golden color slowly whisk in the broth and milk, and lastly, whisk in the salt and pepper. If your creamy sauce is too thick whisk in more water until your desired consistency. Set to the side. If you want to skip this step, you can use a 10 oz can of cream of chicken soup instead.
To a large bowl, add the stuffing and 4 tbsp of melted butter and stir well to coat the stuffing. Remove 3/4 cup of the stuffing and set aside. This will be the topping for the end.
To the bowl with the majority of the stuffing, add in the creamy sauce, zucchini, onion, chicken, sour cream, salt, and pepper. Stir well to incorporate the ingredients.
Spray a 9x13 baking dish with non-stick spray. Spread the casserole evenly in the baking dish and top with the reserved stuffing. Bake for 40-45 minutes. Enjoy!