Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

In a very large mixing bowl, combine the can of cream of chicken soup, the full container of sour cream, and the 1/2 cup of melted butter. Whisk thoroughly until the mixture is smooth and evenly combined.

Stir in the finely chopped yellow onion (if using), 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Mix well. Gently fold in the 2 cups of shredded cheddar cheese.

Add the fully thawed hash brown potatoes to the liquid mixture. Use a large spoon or spatula to fold the hash browns into the creamy base until they are evenly coated. Ensure the mixture is well combined without overworking the potatoes.

Pour the entire hash brown mixture evenly into the prepared 9x13 inch baking dish. Spread the mixture out to the edges using the back of a spoon.

Bake in the preheated oven for 40 to 45 minutes, or until the casserole is hot, bubbly around the edges, and the top is lightly golden brown.

While the casserole is baking, prepare the topping. In a small bowl, combine the 1/4 cup of melted butter and the 2 cups of crushed corn flakes (or crackers). Toss until the corn flakes are completely coated in butter.

After the initial 40-45 minute baking period, carefully remove the casserole from the oven. Sprinkle the buttered corn flake mixture evenly over the top of the hot casserole, followed by the remaining 1/2 cup of shredded cheddar cheese.

Return the casserole to the oven and bake for an additional 10 to 15 minutes, or until the topping is golden brown and crispy and the cheese has melted completely.

Remove the casserole from the oven and let it rest for 5 to 10 minutes before serving. This allows the casserole to set and makes serving easier.