Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish with butter or cooking spray.

In a large mixing bowl, combine the cream of chicken soup, sour cream, and melted butter. Whisk until well incorporated.

Stir in chopped onion, sharp cheddar cheese, salt, and pepper into the creamy mixture.

Gently fold in the frozen shredded hash browns, ensuring they are mixed without losing their texture.

Spread the mixture evenly into the prepared baking dish.

Combine crushed cornflakes with melted butter, then sprinkle evenly over the casserole.

Bake uncovered for 45-55 minutes until the top is golden brown and bubbly.

Let the casserole rest for about 10 minutes before serving. Garnish if desired.