1. Put the chicken thighs in a saucepan. Pour over the chicken stock and cider, then add 1 teaspoon of the table salt. Bring to a simmer over a medium heat and cook for 15 minutes. Add the mushrooms to the pan and cook for a further 5 minutes.
2. Remove the pan from the heat and strain the stock through a colander into a second clean pan. Set aside the chicken and mushrooms to cool slightly. Once cool, slice each chicken thigh into three.
3. Place the pan of stock over a medium heat, skim any excess fat from the surface with a spoon every 10 minutes as it comes back to a simmer, and cook until the stock has reduced by half.

Steps 4 to 8 could be done before anything else if you prefer

4. While the stock is reducing, prepare the pastry case. On a lightly floured surface, roll out the pastry to a 5mm thick large rectangle. Cutting diagonally from corner to corner, cut a pastry strip 20cm wide and at least 50cm long, squaring off the ends. Lay the pastry strip on a large piece of parchment paper, folding the pastry if necessary, and chill in the refrigerator until needed.
5. Unclip the pie mould and take it apart. Using the shape of the pie mould as a guide, cut out two ovals from the resulting triangles of pastry, reserving any trimmings for decoration.
6. Re-assemble the pie mould and secure it with the clips. Brush the inside of the mould with the softened butter. Remove the strip of pastry from the refrigerator, gently fold it into a loop and carefully place it inside the greased mould. Let the bottom 2cm of the pastry strip slide down onto the base of the mould and flatten down.
7. Next, press the pastry strip against the sides of the mould following the fluted shape. There should be a substantial overhang of pastry at the top of the mould, which you can ease over the top edge by making small snips in the pastry at either pointed end of the mould. After pressing the sides in, make sure the seam where the two ends of the pastry strip meet is no more than a 2cm overlap. Trim any excess pastry with scissors, if necessary, then push the ends firmly together.
8. Take one of the pastry ovals, trim 1cm from all the way round the edge and lay it on the base of the mould. Push the pastry oval against the mould and into the pastry strip where it overlaps the base to join the two. Put the pastry-lined mould into the refrigerator with the remaining pastry oval and any trimmings for decoration and chill for 30 minutes.
9. Wipe clean the first pan. Put the butter and flour together in the pan and warm over a medium heat. When the butter starts to bubble, reduce the heat to low and cook for 6 minutes, stirring frequently. Gradually add the reduced stock to the butter and flour, one ladleful at a time, stirring well between each addition to stop any lumps from forming and allow the sauce to thicken. Continue until all the stock has been incorporated. Add the double cream and mustard to the sauce and bring it back to a simmer and then cook for a further 4 minutes. Taste and if necessary add more seasoning.
10. Fold the chicken pieces and mushrooms into the sauce and spread the pie filling mixture over a roasting tray to cool quickly. Once the pie filling has cooled to room temperature, pop the tray into the refrigerator for 10 minutes to cool further. Remove the tray from the refrigerator and fold the chopped tarragon into the pie filling.
11. Preheat the oven to 190°C fan/210°C/gas mark 6
12. Remove the pastry-lined mould from the refrigerator. Spoon the chilled pie filling into the pastry case and level the surface. Lay the remaining pastry oval over the top of the filling and lightly brush with the egg wash. Crimp together the lid and sides of the pastry case so the pie is sealed all the way round. Lightly brush again with the egg wash, this time brushing the crimped edges too. Return the mould to the refrigerator and chill the pie for 20 minutes.
13. Remove the mould from the refrigerator and cut a 2cm hole in the middle of the pie lid to allow steam to escape. Decorate the top of the pie however you prefer Lightly brush the pastry all over with egg wash and then return to the refrigerator to chill for 20 minutes. When ready to bake, remove the pie from the refrigerator and brush again with the egg wash.
14. Place the mould on a rack in the centre of the preheated oven and bake the pie for 45 minutes or until the pastry is golden brown. Halfway through the cooking time, turn the tin around in the oven to ensure an even bake. Take the pie out of the mould and serve with some simple steamed broccoli.