Turn over to 350 degrees. Mix sour cream, 2 cups of cheese, cream of chicken soup, onions, and optional chopped green onions. Add defrosted hash browns and mix thoroughly together. Pouring into baking dish and cook for 40 minutes. Crush 2 cups of corn flakes, mix separately with butter and remaining 1/2 cup cheese. When done baking, add corn flake mixture to top and cook for an additional 10 minutes.