Cover the potatoes with boiling water, sprinkle with 1 teaspoon of salt, cover with a lid, and bring to a boil. Cook, covered, over medium heat until the potatoes are soft.

Preheat the oven to 390°F (200°C).

Drain most of the water and pour the potatoes into a stand mixer bowl (or mash them manually with a potato ricer). Add salt, butter, cream, and nutmeg. Mix well and whip until you get a silky, creamy texture.

Transfer the mixture to a sheet pan lined with parchment paper. Sprinkle a generous layer of mozzarella on top and bake until golden, about 20-25 minutes.