Preheat the oven to 350°F (175°C).

In a large mixing bowl, combine the thawed hashbrowns, sour cream, cream of chicken soup, 1.5 cups of cheddar cheese, diced onion, garlic powder, salt, and pepper. Mix until well combined.

Spread the mixture evenly in a greased 9x13 inch baking dish.

In a small bowl, mix the crushed cornflakes with melted butter. Sprinkle the mixture over the hashbrown mixture in the baking dish.

Bake in the preheated oven for 45 minutes or until the top is golden brown and the casserole is bubbly.

Remove from the oven and let it cool for a few minutes before serving. Top with remaining cheddar cheese if desired.