Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the thawed hash browns, shredded chicken, cheddar cheese, cream of chicken soup, sour cream, milk, garlic powder, onion powder, salt, and black pepper. Mix until well combined.
If using, fold in the chopped green onions for added flavor.
Pour the mixture into the prepared baking dish and spread it evenly.
If desired, sprinkle crushed cornflakes on top for added crunch.
Bake in the preheated oven for 30-35 minutes, or until the casserole is bubbly and golden brown on top.
Remove from the oven and let it cool for about 5 minutes before serving.