Preheat oven to 350 degrees. Coat a 13 x 9-inch baking pan or glass pan with cooking spray.
In an extra-large bowl, combine stuffing mix and ⅓ melted butter. Set aside ½ cup of mixture for topping.
In a microwave-safe large bowl, add zucchini, mushrooms, and onions and 1 tablespoon butter. Cook in the microwave for 2 minutes to soften vegetables.
Add cooked veggies to the large bowl of stuffing mixture. Stir in cooked chicken, soup, sour cream, cottage cheese, sage, and pepper. Mix until completely combined.
Place stuffing mixture into baking pan and spread out evenly. Sprinkle reserved ½ cup stuffing mix on top. Bake uncovered for about 45 minutes until golden brown.
Cool slightly and cut into 8 slices. This casserole freezes great!