Grease a 9x13-inch baking dish with cooking spray or butter.
In a large bowl, combine thawed hash browns, cream of chicken soup, sour cream, 2 cups of the shredded cheese, diced onion, garlic powder, salt, and pepper. Mix until everything is well combined.
Spread the mixture evenly into the prepared baking dish.
In a small bowl, combine crushed cornflakes with melted butter and the remaining 1 cup of cheese. Sprinkle this mixture evenly over the top of the casserole.
Cover tightly with aluminum foil and refrigerate overnight (or at least 4 hours).
When ready to bake, preheat oven to 350°F. Bake covered for 30 minutes.
Remove foil and bake an additional 15 minutes until the top is golden brown and crispy.
Let stand for 5 minutes before serving. Garnish with chopped green onions or fresh parsley if desired.