Thaw the potatoes: Move the bag of hash browns to the fridge the night before, or spread them on a sheet pan and leave them at room temperature for about 30 minutes. Pour off any water that collects. Frozen potatoes chill the whole casserole and add 20 minutes to the bake.

Cook the onion: Melt 4 tablespoons of the butter in a small skillet over medium heat. Add the chopped onion and cook for 4-5 minutes, stirring, until it turns soft and translucent with no crunch left.

Mix the base: Heat the oven to 350°F and grease a 9x13 baking dish. In a large bowl, stir together the sour cream, cream of chicken soup, the buttered onion with all the butter from the pan, the salt, pepper, garlic powder, and 2 cups of the cheddar.

Fold in the potatoes: Add the thawed hash browns and fold gently until every piece is coated. Spread the mixture into the baking dish in an even layer and scatter the remaining 1/2 cup of cheddar over the top.

Make the topping: Put the cornflakes in a zip-top bag and crush them with your hands into rough pieces, not crumbs. Toss them with the remaining 4 tablespoons of melted butter until the flakes look damp, then spread them over the cheese.

Bake: Bake uncovered for 45-50 minutes, until the edges bubble and the cornflakes are deep golden. The center should read 165°F on a thermometer. Tent the pan with foil if the topping browns before the middle is hot.

Rest and serve: Let the casserole stand for 5-10 minutes before scooping so it sets up enough to hold its shape on the plate.