Mashed-Potato Casserole

Ingredients

• 4 large potatoes, peeled and cubed
• 1/2 cup butter
• 1/2 cup sour cream
• 1/2 cup milk
• 1 teaspoon garlic powder
• Salt and pepper to taste
• 3/4 cup shredded Cheddar cheese, divided
• 1/4 cup chopped fresh chives

Directions

1. Fill a large pot with generously salted water and bring it to a boil. Drop in the potatoes and cook them until a metal skewer slips in easily, 15 to 20 minutes. Drain them thoroughly, then put them back in the pot, setting it over very low heat.
2. Pour in the milk and mash until the potatoes are smooth. Slice 5 Tbs. of the butter into pieces and add it to the pot with the crème fraîche, pepper, and nutmeg. Stir until the butter disappears into the potatoes, then take the pot off the heat. Set aside 1/2 cup of the cheese, add the remaining cheese to the potatoes, and stir it in. Add 1/2 tsp. salt and a pinch of pepper for seasoning.
3. Spoon the potatoes into a square 9-inch baking dish, or choose a casserole dish about the same size. If you are baking now, place an oven rack in the middle and heat the oven to 400°F; if you are waiting, start the oven about 25 minutes before baking.
4. In a bowl, mix the panko with the reserved 1/2 cup cheese and 1/2 tsp. salt. Take the remaining 3 Tbs. of butter, cut it into pieces, and rub it into the panko with your fingers until it is spread through the crumbs; it should look a little uneven. Scatter the panko mixture over the potatoes. At this stage, you may cover the casserole and refrigerate it for up to 24 hours.
5. Bake until the potatoes are hot all the way through and the topping is golden-brown; depending on how cold the casserole is when it starts, this will take 15 to 35 minutes. If the top needs more color, give it a short turn under the broiler. Serve hot or warm, with sea salt sprinkled over the top if you like.