Preheat the oven to 350 degrees F (175 degrees C). Spray a 9x13-inch casserole dish with cooking spray.
Drain defrosted hash browns well. Combine drained hash browns, 2 cups Colby cheese, 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 ½ cup of cheese over the top.
Bake in the preheated oven until cheese is melted and starts to brown, 45 to 55 minutes.
Garnish with chopped parsley and serve hot.
