Preheat the oven. Preheat the oven to 350°F. Grease a 9x13-inch baking dish.
Soften the onion. Melt the 1/2 cup butter. Add about 1 tablespoon of the melted butter to a small skillet, then add the diced onion. Cook over medium heat for 4 to 5 minutes, until softened.
Mix the creamy base. In a large mixing bowl, stir together the cream of chicken soup, sour cream, remaining melted butter, salt, pepper, garlic powder, and onion powder.
Add the potatoes and cheese. Fold in the thawed diced hash browns, cooked onion, and shredded cheddar cheese until evenly combined.
Assemble the casserole. Spread the potato mixture evenly into the prepared baking dish.
Make the topping. In a separate bowl, stir together the lightly crushed cornflakes and 4 tablespoons melted butter.
Top the casserole. Sprinkle the buttered cornflakes evenly over the potatoes.
Bake. Bake uncovered for 50 to 60 minutes, or until the casserole is hot and bubbling around the edges and the topping is golden brown.
Rest before serving. Let the casserole rest for 10 minutes before serving.