Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the thawed hashbrowns, condensed cream of chicken soup, sour cream, melted butter, chopped onion, 1 1/2 cups of the shredded cheddar cheese, salt, black pepper, garlic powder, and onion powder. Mix well until all the ingredients are evenly distributed.
Pour the mixture into a 9×13-inch baking dish, spreading it out evenly.
Sprinkle the remaining 1/2 cup of shredded cheddar cheese over the top of the casserole. If using, sprinkle the crushed cornflakes or breadcrumbs evenly over the cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and crispy and the casserole is heated through.
Remove the casserole from the oven and let it cool for a few minutes before serving.