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In the morning - preheat the oven to 220C/gas 7/fan 200C and bake the potatoes for about 1 hour until the insides are soft.
Whisk the egg yolks, cream, 3 tbsp horseradish and 1 tbsp lemon juice in a large bowl. Cut the potatoes in half and spoon the insides into the bowl, reserving the skins.
Mash the potato and flavourings together, then season with salt and a little pepper. Taste and add more horseradish and/or lemon juice if required. Pile the potato back into the skins, then place the potatoes on a baking tray. Loosely cover with cling film and keep them cool, not chilled.
Half an hour before serving - dot the filling with scraps of butter and bake in the oven with the pie for 25-30 minutes until the tops are browned.