Preheat your oven to 350 degrees Fahrenheit. Grease a 9×13 inch baking dish.
In a large bowl, combine the thawed hashbrown potatoes, cream of chicken soup, sour cream, melted butter, chopped onion, salt, and pepper. Mix until well combined.
Stir in 1 1/2 cups of the shredded cheddar cheese into the potato mixture.
Spread the mixture evenly into the prepared baking dish.
In a small bowl, mix the remaining 1/2 cup of cheddar cheese with the crushed corn flakes, if using. Sprinkle this topping evenly over the casserole.
Bake for 45 to 55 minutes, or until the casserole is bubbly throughout and the topping is golden brown.
Let the casserole rest for 5 minutes before serving.
Notes
You can prepare this casserole the night before and store it covered in the refrigerator. Add 10 minutes to the baking time if cooking directly from cold.
For a richer flavor, substitute cream of chicken soup with cream of mushroom soup.
Use frozen, uncooked hashbrowns for the best texture; do not use pre-cooked or dehydrated potatoes.