Preheat the oven to 375 degrees Fahrenheit and spray a 9x13-inch baking dish with non-stick cooking spray. Place the 14 ounces sliced kielbasa sausage, and 32 ounces frozen pierogies in the prepared baking dish.
In a bowl, using a hand mixer, whisk together the 1 (8-ounce) block softened cream cheese and ½ cup sour cream until smooth. Add in the ½ cup chicken broth, 1 teaspoon onion powder, 1 teaspoon garlic powder, ½ teaspoon salt, ½ teaspoon pepper, HALF of the 1 cup shredded cheddar cheese, and HALF of the 1 cup mozzarella cheese, and whisk until well combined.
Pour the sauce over the pierogies and sausage and mix to coat everything well. Top the dish with the remaining cheddar cheese and mozzarella cheese.
Cover the dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 10 minutes, until the top is bubbly and golden.
Remove from the oven and let the casserole cool for a few minutes before topping with sliced green onions for garnish and serving.