Put the chicken thighs, mushroom and bay leaves in a large saucepan. Pour over the sock and season well. Place the pan over a medium high heat and bring the liquid to the boil.
Cover with a lid, reduce the heat and simmer for 25 minutes, until the chicken thighs are cooked and tender. Set aside to cool.
While the chicken is cooking, in a separate saucepan add boiling salted water, boil the celery and carrots for about 5 minutes, until they are tender. Use a slotted spoon to remove the celery and carrots and transfer them to a bowl.
Add the leeks to the same cooking water and boil for about 5 minutes, until tender. Drain and set aside to cool.Once the chicken has cooled, remove the thighs from the stock and tear the meat from the bones, pulling it off in bite-sized pieces and transferring it to a bowl as you go.
Pour the chicken stock through a sieve, reserving it in a large jug. You should have about 500ml. If you don’t have enough, make up the volume with a little extra chicken stock from a stock cube, or with white wine.
Heat the oil in a large saucepan over a low heat. When hot, add the onion, season with salt and pepper and fry for 10 minutes, until the onion is softened but has not taken on any colour.
Add the butter and stir until melted, then add the flour and stir to form a smooth paste. A little at a time, pour the reserved stock into the paste, whisking as you go. Keep whisking over a medium heat, until the mixture has thickened to a smooth sauce consistency.
Stir in the crème fraiche and mustard powder, then add the chicken, celery, carrots, leeks and finally the tarragon, stirring everything through to evenly combine. Season to taste.
Spoon the filling into a 2-litre ovenproof pie dish and level the top. Leave until cool, then cover and refrigerate for 30 minutes.
Preheat the oven to 200°C/180°C fan/Gas 6.While the filling is in the fridge, prepare the pastry. On a lightly floured work surface, unroll the pastry sheet, and roll it if necessary to about 3mm thick, making sure it will fit over the pie dish with about a 2cm overhang.
12. Remove the chilled filling from the fridge. Brush the edge of the pie dish with the beaten egg and gently transfer the pastry lid over the top, pressing around the edges to seal. Trim to neaten and use the trimmings to add a 1cm border around the edge of the pie, which will help prevent shrinkage.
Brush the pastry with the remaining beaten egg and make an incision in the middle of the pie with a sharp knife to let out the steam. Bake for 30 minutes, until the pastry is golden and the filling is bubbling hot.