Preheat your oven to 350°F and generously butter a 9x13-inch baking dish. Thaw your hashbrowns completely and squeeze out excess moisture with clean kitchen towels.

In a large mixing bowl, combine sour cream, cream of chicken soup, 1½ cups of shredded cheese, melted butter, diced onion, garlic powder, salt, and pepper. Whisk until smooth and well-blended.

Add the thawed hashbrowns to your creamy mixture and fold gently until every strand is coated. Taste and adjust seasoning if needed.

Spread the hashbrown mixture evenly in your prepared baking dish, pressing down lightly with the back of a spoon.

In a small bowl, toss crushed cornflakes with 3 tablespoons melted butter until evenly coated. Sprinkle this mixture over the casserole, followed by the remaining ½ cup of cheese.

Bake for 40-45 minutes until the edges are bubbling and the top is golden brown. The center should feel set when gently pressed. Let rest for 5 minutes before serving.