Preheat your oven to 350°F and lightly grease a 9×13-inch baking dish.
In a large mixing bowl, combine the thawed hash browns, sour cream, cream of chicken soup, 1 1/4 cups of shredded cheddar cheese, 3 tablespoons of melted butter, the chopped onion, salt, dried parsley, and garlic powder. Stir until everything is evenly mixed.
Spread the potato mixture evenly into the greased baking dish.
In a small bowl, mix the crushed corn flakes with the remaining 1 cup of cheddar cheese and the remaining 2 tablespoons of melted butter. Sprinkle evenly over the casserole.
Place the dish in the preheated oven and bake for 45 minutes, until bubbly and golden brown.
Remove from the oven and let cool for 5 minutes before serving.
