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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.
To make the pie, unroll the pastry rectangle onto a baking sheet. Pile the filling along the length of one half of the pastry, leaving a small gap on the outside edge. Brush milk round the edges of the pastry. Fold the uncovered side over the filling, then press the pastry edges together well, using the prongs of a fork, to seal. Make a few 6cm diagonal slashes in the top. (The pie can now be frozen for up to a month.)
To bake the pie, brush the pie with the remaining milk and bake for 30 minutes until golden brown. Leave to relax for a couple of minutes then cut into slices and serve with a green salad.