Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the thawed hashbrowns and chopped onion.
Add the cream of chicken soup and sour cream to the bowl.
Stir in melted butter, salt, and pepper until well mixed.
Fold in the shredded cheddar cheese.
Transfer the mixture to a greased 9x13 inch baking dish.
If using, sprinkle crushed cornflakes over the top.
Bake for 45 minutes, or until golden brown and bubbly.
Let it cool for a few minutes before serving.
Enjoy your delicious hashbrown casserole!