Preheat oven to 350°F (175°C). Grease a 2 1/2-quart casserole dish.

In a large bowl, combine thawed hash brown potatoes, melted butter (1/2 cup), chopped onion, undiluted cream of chicken soup, sour cream, and grated Cheddar cheese.

Stir the mixture well to ensure all ingredients are evenly distributed.

Pour the potato mixture into the prepared casserole dish.

In a separate small bowl, toss the crushed cornflakes with the remaining melted butter (1/4 cup).

Sprinkle the buttered cornflakes evenly over the potato mixture in the casserole dish.

Bake in the preheated oven for 50 minutes, or until golden brown and bubbly.

Let the casserole stand for a few minutes before serving.

Serve hot and enjoy!