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Cook the potatoes in a pan of boiling salted water for 15 mins until really tender when pressed into the side of the pan. Drain well and leave to steam-dry in the pan for 2 mins. Mash the potatoes until very smooth – use a ricer if you have one, or use a masher, then press the potatoes through a metal sieve to ensure they're completely lump-free.
Mix the egg yolk, butter and cheese in a large bowl, then add the mash, nutmeg and some seasoning. Mix well with a spatula until the butter has melted and is well incorporated. It will come away from the sides of the bowl, similar to choux pastry.
Heat the oven to 220C/200C fan/gas Spoon the mash into a piping bag with a wide star nozzle. Line a baking sheet with baking parchment and brush over a little vegetable oil. Pipe 20 walnut-sized peaked mounds onto the prepared tray, as if you were icing a cupcake. Beat the egg white with a pinch of salt and brush the potato mounds gently, to keep the fluting.
Bake the potatoes for 15-20 mins until golden-brown and crisp.