Preheat the over to 350 degrees F (175 degrees C). Spray a 9x13-inch casserole dish with cooking spray.
Drain defrosted hash browns well (I used a salad spinner). Combine drained hash browns, 2 cups Colby cheese, cream of chicken soup, sour cream, onion, butter, egg, garlic powder, salt, and pepper in a large bowl.
Spoon the mixture into the casserole dish, and sprinkle the remaining 1/2 cup of cheese over the top.
Bake in the preheated oven until cheese is melted and starts to brown, 45 to 55 minutes.