Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the thawed hashbrowns, sour cream, cream of chicken soup, 1 1/2 cups of cheddar cheese, chopped onion, melted butter, garlic powder, salt, and black pepper. Mix until well combined.
Pour the mixture into a greased 9x13-inch baking dish, spreading it evenly.
If using, sprinkle the crushed cornflakes evenly over the top of the casserole for added crunch.
Bake in the preheated oven for 45-50 minutes, or until the casserole is bubbly and the top is golden brown.
Remove from the oven and let it cool for about 5-10 minutes before serving.