To make the pastry, mix the flour with the salt, then add the cubed butter and toss until the pieces are coated with flour. Add just enough iced water (8–10 tablespoons) to bring the mixture together into a fairly firm dough.
Shape the dough into a rectangle with your hands and, on a well-floured surface, roll it out in one direction, away from you, so you end up with a rectangle about 1cm thick. Fold the far third towards you, then fold the nearest third over that (rather like folding a business letter), so that you now have a rectangle made up of 3 equal layers. Give the pastry a quarter-turn, then repeat the rolling, folding and turning process 5 more times. Wrap the pastry in a wax wrap or baking paper and rest it in the fridge for about 30 minutes, or up to an hour.
To make the filling, melt the butter in a frying pan, add the leeks and thyme, and sweat gently for 5–10 minutes, until the leeks are very tender. Stir in the cream and cook gently for 4–5 minutes to reduce and thicken. Stir in the mustard, season well and leave to cool.
If using fresh chicken, season well and fry in the oil in a frying pan over a medium-high heat until evenly browned and cooked through, then leave to cool.
Roll out the pastry on a lightly floured work surface to about 3mm thick. Using a plate or a cake tin as a template, cut out four 20cm circles; you may have to gather up the trimmings, lay them on top of each other, and re-roll them to get your fourth circle.
Spoon the leek mixture on to one half of each circle and pile the chicken on top. Brush the pastry edges with a little water, fold the other half of the pastry over the filling to form a half-moon shape and crimp well to seal.
Place on a lightly oiled baking sheet and brush the tops of the pasties with beaten egg. Bake in an oven preheated to 190°C/Gas Mark 5 for about 25 minutes, until the pastry is golden brown. Eat hot or cold.
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