Photograph: Jonathan Gregson
Gently cook the leeks and shiitake mushrooms in the butter until tender, then cool. Add the chicken, tarragon and parsley, and season. Set aside.
For the sauce, melt the butter in a pan, add the flour and cook over a low heat for 1-2 minutes until the mixture is golden and smells biscuity. If you don't cook the butter and flour properly, your sauce will taste floury. Pour in the stock and turn up the heat, whisking constantly, until it reaches simmering point and thickens slightly. Off the heat, stir in the cream, mustard and lemon juice, then cool.
Mix the sauce with the pie filling and check the seasoning. Preheat the oven to 200°C, fan 180°C, gas 6.
For the pastry, put the flour and ½ teaspoon of salt on a sheet of baking paper (or in a jug). In a small pan, bring 50ml water, the milk and butter to the boil, then shoot the flour into the pan. Beat until lump free and shiny. You may need a little more hot water if the mixture doesn't work itself together properly into a smooth dough. Allow to cool to a manageable temperature, then, on a lightly floured surface, roll out the pastry dough to a thickness of ¼ cm.
Add the filling to a 25 x 18cm (1.5 litre) pie dish and drape the pastry over the top of the dish. Using a fork, press down along the edge of the dish, then carefully trim away the excess pastry.
Use a sharp knife to cut 2 holes in the pastry top to allow the air to escape. Brush the pie with the beaten egg and bake for 30-40 minutes until the top is golden and the filling piping hot. Remove the pie from the oven and let it stand for 5 minutes or so before serving.
Get ahead: assemble the pie, ready to cook, a few hours ahead; chill.
Kitchen secret: if you don’t have time to make the pastry, use ready-made puff pastry, which will make a perfectly tasty pie. Fill the pie dish with the filling, then brush the edge of the dish with the beaten egg. Cut the pastry to fit before placing on top of the filling and finishing the recipe as described.