In a large mixing bowl, combine the thawed shredded hashbrowns, cream of chicken soup, and melted butter. Stir in the chopped onion, shredded cheddar cheese, sour cream, salt, and pepper. Mix until evenly incorporated.

Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking dish by lightly spraying it with non-stick cooking spray. Pour the hashbrown mixture into the dish, spreading it out evenly.

Sprinkle the reserved cheddar cheese over the top of the mixture in the baking dish.

Bake for 45 to 50 minutes until the top is golden brown and the edges are bubbling.

Once baked, allow the casserole to cool for about 5 minutes before serving.