Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or nonstick spray. In a large bowl, mix together the thawed hashbrowns, cream of chicken soup, sour cream, cheddar cheese, onion, melted butter, salt, pepper, and garlic powder until well combined.
Spread the mixture evenly into the prepared baking dish. In a small bowl, toss the crushed cornflakes with ¼ cup melted butter. Sprinkle over the casserole.
Bake uncovered for 45–50 minutes, or until the top is golden brown and the casserole is hot and bubbly.
Let rest 5–10 minutes before serving.
