Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish.
In a large bowl, combine the thawed hash browns, sour cream, melted butter, cream of chicken soup, diced onion, half of the cheddar cheese, salt, and pepper. Mix well until everything is evenly combined.
Spread the mixture evenly into the prepared baking dish.
Bake for 45-60 minutes, or until golden brown and bubbly. About 10 minutes before it's done, sprinkle the remaining cheddar cheese over the top.
Let it rest for a few minutes before serving.
Notes
This casserole can be made ahead of time and refrigerated. Bake as directed, adding a few extra minutes if baking from cold.