Preparation

• 一锅well-salted水煮沸。Add the potatoes, and cook until easily pierced with a metal skewer, 15 to 20 minutes. Drain well and return to the pot over very low heat. Add the milk, and mash until smooth. Cut 5 Tbs. of the butter into pieces. Add to the potatoes along with the crème fraîche, pepper, and nutmeg, and stir until the butter has melted. Remove from the heat. Reserve 1/2 cup of the cheese, and then add the rest to the potatoes. Stir to combine. Season with 1/2 tsp. salt and a pinch of pepper. Transfer to a 9-inch square baking dish or similar-size casserole dish. If baking right away, position a rack in the center of the oven and heat the oven to 400°F; otherwise, heat the oven about 25 minutes before baking.
• In a bowl, combine the panko, the reserved 1/2 cup cheese, and 1/2 tsp. salt. Cut the remaining 3 Tbs. of butter into pieces, then use your fingers to work it into the panko, dispersing it evenly; the mixture will be lumpy. Top the potatoes with the panko mixture. At this point, the casserole may be covered and refrigerated for up to 24 hours.
• Bake until the potatoes are heated through and the topping is golden-brown; the time will vary depending on the starting temperature of the casserole, 15 to 35 minutes. To speed up browning, broil briefly, if necessary. Serve hot or warm, sprinkled with sea salt, if you like.