In a blender, put together flour,butter, and a pinch of salt and pulse until the mixture is sandy-looking.
Transfer the mixture into a large bowl. Knead quickly while gently pouring cold water in, until the dough is compact, firm and elastic enough.
Shape the dough into a rectangular, and wrap in plastic film. Let refrigerate for at least 40 min.
For the chicken and mushroom filling:
In a very large skillet heat the olive oil, add the sliced leeks and stir-fry for about 5 min, or until crisp.
Add the chicken, searing well the meat on all sides; Finally, add the mushrooms. Pour the white wine in and season with salt and pepper. Allow the alcohol to evaporate, then pour ⅓ cup of chicken stock in, and simmer until chicken is cooked through.
In a saucepan melt the butter and add the flour, stirring with a whisk.
Pour in the remaining chicken stock, stirring constantly to prevent lumps from forming. Thicken the cream over low heat and then add it to the chicken.
Add fresh thyme leaves and double cream, cook over low heat for about 5 min,until the filling has thicken enough.
For the chicken and mushroom pies:
Roll out the refrigerated shortcrust pastry into ⅓ inch high sheet over a clean surface sprinkled with flour. Cut out 16 10cm circles and use half to line 8 non-stick tart tins. Reserve the remaining pastry circles to top the pies.
Pour 2-3 generous tbsp of chicken&mushroom mixture into the center of each pie, forming a mound.
Cover each pie with a spare pastry circle, pressing the edges gently to firmly adhere and prevent from leaking.
Brush the pies with the egg and water mixture and sprinkle with a little sea salt.
Bake in a preheated oven to 190 ° C for about 50 minutes, until the pies are golden on top. Allow to rest 10 minutes outside the oven before serving.