Preheat your oven to 375°F (190°C). Lightly grease a standard 9x13 inch baking dish.

Ensure the frozen hashbrown potatoes are completely thawed and gently patted dry with paper towels to remove excess moisture. If you are using the crisp topping, combine the crushed cornflakes (or panko breadcrumbs) with 1/4 cup of reserved melted butter in a small bowl and set aside.

In a very large mixing bowl, whisk together the condensed cream of chicken soup, sour cream, and the remaining melted butter (about 1/4 cup) until the mixture is smooth. Add the diced yellow onion, garlic powder, salt, and black pepper. Stir well.

Gently fold the shredded or diced cooked chicken into the soup mixture. Add the thawed hashbrown potatoes to the bowl. Use a large spoon or spatula to carefully fold the hashbrowns into the mixture until they are evenly coated. Do not overmix, which can make the potatoes gummy.

Pour the entire hashbrown and chicken mixture into the prepared 9x13 inch baking dish, spreading it evenly across the bottom. Sprinkle 1 and 1/2 cups of the shredded cheddar cheese evenly over the top of the casserole mixture.

If using the optional crisp topping, sprinkle the cornflake/butter mixture evenly over the layer of cheese. Bake the casserole in the preheated oven for 40 minutes.

After 40 minutes, remove the casserole from the oven. Sprinkle the remaining 1/2 cup of shredded cheddar cheese over the top. Return the casserole to the oven and bake for an additional 5 to 10 minutes, or until the cheese is melted, bubbly, and the edges of the casserole are golden brown.

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