Boil potatoes in a saucepan with water and a pinch of salt for about 20 minutes. Drain and return to the pan.
Fry bacon until golden and crisp, then set aside on a paper towel.
Mash the potatoes with milk, sour cream, butter, salt, pepper, nutmeg, and grated Parmesan until smooth.
Spread the mashed potatoes in a baking tin, sprinkle with mozzarella, Cheddar, and Parmesan.
Cut the bacon into small pieces and sprinkle on top of the casserole.
Bake at 380°F (190°C) for 20-25 minutes.
Garnish with chives before serving.
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