Preheat the oven to 350°F / 175°C and grease a 9x13-inch baking dish.
Combine the thawed hashbrowns, 1.5 cups cheddar cheese, sour cream, cream of chicken soup, melted butter, onion, garlic powder, and black pepper in a large bowl until evenly mixed.
Spread the mixture evenly into the prepared baking dish.
Bake uncovered for 40 minutes until the edges are bubbling.
Sprinkle the remaining 1/2 cup cheddar cheese on top and bake for 5 more minutes until melted and golden.
Let the casserole rest for 5 minutes before serving so it sets properly.