To make the filling, preheat the oven to 220C/gas 7/fan 200C. Slice your potato leftovers as best you can, then blanch the leek in boiling water for 3 minutes and drain. If you don’t have any leftovers, peel, halve and thickly slice a baking potato, then boil for 8 minutes, adding the sliced leek for the last 3 minutes. Drain. In a large bowl, mix the potato, leek, horseradish sauce, crème fraîche, ham and thyme, mushing up the potato slightly, then season well.