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Heat oven to 180C/160C fan/gas 5. Season the chicken and roast for 1 hr 30 mins until the juices run clear when you cut into the leg.
To make the pastry, sift the flour and salt into a bowl, then grate in the butter and lard. Rub the mixture with your fingers into breadcrumbs, then add 2 tbsp cold water. Continue to add cold water, 1 tbsp at a time, until the mixture binds together. Be careful not to add too much. Wrap in cling film and leave to rest while you make the filling. (Alternatively you can use ready-rolled shortcrust pastry).
In a large, non-stick frying pan, heat the oil, then fry the bacon. Once browned, reduce the heat, add the leeks and cook until they have softened. Mix the butter and flour into a paste and add to the pan, stirring to coat the bacon and leeks. Slowly add the milk, stirring constantly to ensure the mixture doesn’t get lumpy. Add the chicken stock and petit pois, cook for 2 mins, then add the mustard and remove from the heat. Once the chicken is cooked, remove from the oven, leave to rest briefly and carve the meat, discarding the skin and bones. Add this to the pan along with the parsley.
Turn the oven up to 220C/200C fan/gas 6. Roll out two-thirds of the pastry and use to line a 25cm pie dish. Spoon your filling on top, then roll out the remaining pastry for the pie topping. Brush the edges of the pastry in the pie dish with water, then lay the top over it. Crimp and seal the edges and use any off-cuts for decoration. Brush with the beaten egg, then make an X-shaped cut in the middle for steam to escape. Bake for 45 mins until golden.