Start by preheating your oven to 350 degrees Fahrenheit. Lightly grease a 9 by 13 inch baking dish with butter or a non stick cooking spray to ensure the cheese does not stick to the sides. Ensure your hash browns are fully thawed and patted dry with a paper towel, as excess moisture can make the casserole too soft.
In a large mixing bowl, combine the melted butter, condensed cream of chicken soup, sour cream, and finely diced onions. Whisk these ingredients together until the mixture is smooth and well incorporated. Stir in the salt and black pepper to season the base evenly.
Add the shredded cheddar cheese to the bowl and stir well. Gently fold in the thawed hash browns, making sure every potato shred is thoroughly coated in the creamy sauce. Once mixed, transfer the contents into your prepared baking dish and use a spatula to spread it into an even layer.
In a separate small bowl, toss the crushed cornflakes with the remaining quarter cup of melted butter until the flakes are well coated. Sprinkle this mixture evenly over the top of the potato layer. This topping is what gives the casserole its iconic golden color and satisfying crunch.
Place the dish in the center of the oven and bake for 45 minutes. You will know it is ready when the edges are bubbling and the cornflake topping has turned a beautiful golden brown. Allow the casserole to rest for about 5 minutes before serving to let the sauce set slightly.
