Follow these simple steps to create your Chicken Hashbrown Casserole:

1. Preheat Oven: Preheat your oven to 350°F (175°C).
2. Prepare the Casserole Dish: Grease a 9×13-inch baking dish with cooking spray or butter.
3. Combine Ingredients: In a large mixing bowl, add thawed hashbrowns, cooked chicken, shredded cheddar cheese, sour cream, cream of chicken soup, onion, bell pepper (if using), garlic powder, black pepper, and salt. Mix until well combined.
4. Transfer to Dish: Pour the mixture into the prepared baking dish, spreading it evenly.
5. Prepare Topping: If using cornflakes, mix them with melted butter in a small bowl. Sprinkle the mixture evenly over the casserole.
6. Bake: Place the casserole in the preheated oven and bake for about 45 minutes, or until the top is golden brown and the casserole is bubbly.
7. Cool: Once done, remove from the oven and let it cool for about 15 minutes before serving.
8. Serve: Cut into squares and serve warm.

These easy steps will guide you in creating a delicious Chicken Hashbrown Casserole that is sure to be a hit!