Preheat your oven to 350°F (175°C). Lightly grease your 9x13-inch baking dish with butter or non-stick spray to prevent sticking.
In a large mixing bowl, combine the thawed hash brown potatoes, sour cream, cream of chicken soup, half of the melted butter (¼ cup), sliced green onions (or minced onions), kosher salt, black pepper, and shredded cheddar cheese. Stir everything gently but thoroughly until all ingredients are evenly distributed.
Transfer the potato mixture into the prepared baking dish. Use a spatula to smooth the top so it’s even.
In a separate bowl, crush the cornflakes lightly and mix with the remaining melted butter. This buttered cornflake topping will add a delicious crispy texture once baked.
Sprinkle the buttered cornflakes evenly over the potato mixture. Place the dish in the oven and bake for 45-55 minutes, or until the edges are bubbling and the topping is golden brown.
Let the casserole cool for about 10 minutes before serving. This resting time allows the dish to set, making it easier to scoop out and enjoy.