Preheat your oven to 350°F (175°C). Lightly grease a standard 9x13 inch baking dish. Ensure your hashbrowns are completely thawed and gently squeeze out any excess liquid using paper towels or a clean kitchen towel.

In a large mixing bowl, whisk together the canned cream soup, sour cream, 1/2 cup melted butter, chopped onion (if using), salt, and black pepper until the mixture is smooth and well combined.

Add 1 1/2 cups of the shredded cheddar cheese to the creamy mixture and stir to incorporate. Reserve the remaining 1/2 cup of cheese for the topping later.

Add the thawed hashbrowns to the mixing bowl. Gently fold the hashbrowns into the creamy cheese base using a spatula until the potatoes are evenly coated. Do not overmix or mash.

Pour the entire hashbrown mixture into the prepared 9x13 inch baking dish and spread it into an even layer. Place the casserole in the preheated oven and bake for 30 minutes.

If using the optional topping, combine the crushed crunchy topping (cornflakes/crackers) with 2 tablespoons of melted butter in a small bowl.

After 30 minutes of baking, remove the casserole from the oven. Sprinkle the reserved 1/2 cup of cheddar cheese evenly over the top, followed by the prepared crunchy topping (if using).

Return the casserole to the oven and continue baking for another 15 to 20 minutes, or until the edges are bubbly and the topping is golden brown. Remove and let it rest on a cooling rack for 10 minutes before serving.