Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine thawed hashbrowns, cream of chicken soup, sour cream, 1 ½ cups cheddar cheese, melted butter, onion, garlic powder, salt, and pepper. Mix until evenly combined.
Transfer mixture into the prepared baking dish and spread evenly.
Sprinkle the remaining ½ cup cheese on top. If desired, add crushed cornflakes or breadcrumbs for a crispy topping.
Bake uncovered for 45 minutes, until hot, bubbly, and golden brown on top.
Let rest for 5 minutes before serving.