Preheat the Oven
Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter.
Mix the Ingredients
In a large mixing bowl, combine the cream of chicken soup, sour cream, melted butter, diced onion, salt, and pepper. Stir until well mixed.
Add the Hashbrowns and Cheese
Fold the thawed hashbrowns and shredded cheddar cheese into the mixture until everything is evenly coated.
Transfer to Baking Dish
Pour the hashbrown mixture into the prepared baking dish, spreading it out in an even layer.
Optional Topping
For extra crunch, sprinkle the casserole with the crushed cornflakes mixed with melted butter. Alternatively, you can add a layer of shredded cheese on top.
Bake
Bake the casserole for 45-50 minutes, or until the top is golden and bubbly. Let it sit for a few minutes before serving.