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First, make the pastry. Put the butter in a bowl with the flour and salt. Using your fingertips, rub the butter into the flour until the mixture resembles coarse breadcrumbs. Add about 6 tbsp water to the mixture and use a knife to mix together until you have a firm dough. Cut the dough in half and shape each into a flat disc. Wrap in cling film and put in the fridge while you make the filling.
Put the potatoes in a medium saucepan and cover with water. Bring to the boil and simmer for 5-7 mins until partcooked. Drain well and allow to cool.
Heat the oil in a large saucepan. Add the onion and leeks, and gently cook for 10-15 mins until they are very tender, being careful not to brown them. Add the garlic and cook for a few mins more, then remove from the heat, tip into a bowl and allow to cool. Stir in the chives.
Heat oven to 180C/160C fan/gas 4. Put a baking tray in the oven to preheat. Roll out 1 disc of the pastry on a lightly floured surface to 5mm thick. Use this to line a 24cm round x 4cm deep pie tin, leaving any excess pastry to overhang the tin. Prick the pastry all over with a fork, place on the heated baking tray and bake for 10-12 mins. Return the baking tray to the oven.
Slice the cooled potatoes into very thin rounds. Start to assemble the pie by arranging half of the potatoes in the base, then top with half the leeks, half the crème fraîche dotted over the top and half the cheese. Arrange half the ham on top, then repeat again with all the remaining ingredients.
Roll out the remaining disc of pastry to about a 26cm circle. Brush the top edge of the pastry in the tin with the beaten egg. Lay the circle of pastry over the top and trim the edges, crimping with your fingertips or a fork to seal. Brush all over with the beaten egg and decorate the top with any leftover pastry cut into leaves or a decoration of your choice.
Place on the hot baking tray on the lowest shelf and bake for 45 mins until golden. Set aside for 15 mins to cool in the tin before transferring to a wire rack to cool. Cut into wedges and serve.