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

In a large mixing bowl, whisk together the melted butter, condensed cream of chicken soup, sour cream, diced onion, salt, and black pepper until well combined.

Add the thawed shredded hash browns and 1.5 cups of the shredded cheddar cheese to the bowl.

Fold the ingredients together using a spatula until the hash browns are evenly coated with the cream mixture.

Transfer the mixture to the prepared baking dish and spread into an even layer.

Top the casserole with the remaining 0.5 cups of shredded cheddar cheese.

Bake for 45 to 55 minutes, or until the casserole is bubbling at the edges and the cheese on top has turned a light golden brown.

Remove from the oven and let the dish rest for 10 minutes to allow the sauce to set before serving.