Creating Hashbrown Casserole is simple if you follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Combine Ingredients: In a large mixing bowl, combine the thawed hashbrowns, chopped onion, melted butter, shredded cheddar cheese, cream of chicken soup, sour cream, garlic powder, salt, and black pepper. Mix well until evenly combined.
3. Prepare Baking Dish: Grease a 9×13 inch baking dish with cooking spray or a little butter.
4. Transfer Mixture: Pour the hashbrown mixture into the prepared baking dish, spreading it out evenly.
5. Add Topping: If using, sprinkle crushed cornflakes on top of the mixture for added crunch.
6. Bake: Place the dish in the preheated oven and bake for 45 minutes or until the top is golden brown and bubbly.
7. Cool Slightly: Remove from the oven and let it cool for 5-10 minutes before serving.

These steps will guide you in creating this incredible casserole effortlessly.