Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine frozen hashbrowns, sour cream, cream of chicken soup, shredded cheddar cheese, chopped onions, melted butter, garlic powder, salt, and black pepper. Mix well until all ingredients are evenly combined.
Pour the mixture into a greased 9x13 inch baking dish, spreading it evenly.
If desired, sprinkle crushed cornflakes on top for added crunch.
Bake in the preheated oven for 45 minutes, or until the casserole is bubbly and the top is golden brown.
Once cooked, remove from the oven and let it cool for a few minutes before serving.
Optional: Customize the casserole by adding cooked bacon bits or diced bell peppers for extra flavor.
If making ahead, prepare the casserole, cover it with plastic wrap, and refrigerate it for up to 24 hours before baking. Increase cooking time slightly if baking straight from the refrigerator.
Store leftovers in an airtight container in the refrigerator for up to 3 days and reheat in the oven or microwave.