Place the Flour, Salt and Butter in the bowl of your food processor. Blend until it resemble thin sand or crumbs. Add the cold water and pulse until the dough starts to come together. (see note 1 to do without a food processor)
Transfer onto a lightly floured surface and assemble the dough in a ball. Cover with wrap and gently press on the dough to flatten it (this will help the pastry to chill more uniformly). Place in the fridge to chill.
Chicken and Mushroom Filling
Cut the Chicken Thighs in small pieces.Pour the Olive Oil in a large deep pan. Add the cut chicken thighs with the salt and garlic powder (or crushed garlic). Cook over medium heat. When the chicken pieces start to turn white, cover with the chicken stock and leave to simmer for 5 minutes.
In the meantime, peal and slice the small white mushrooms then add them to the pan.
In a separate bowl, whisk together the heavy cream, cornstarch and curry powder. Pour over the chicken and mushrooms with the dried thyme flakes. Whisk well and leave to simmer on medium heat for 10 to 15 minutes.
When the filling starts to thicken, taste and adjust the salt and spices if required. Add the black pepper powder, stir then remove from the heat and leave to cool down for 10 to 15 minutes.
Putting the Chicken and Mushroom Pie together
Preheat the oven on 350°F.
Take the shortcrust pastry out of the fridge. Cut it in two pieces, one about ⅔ of the dough and the other about ⅓ of the dough.Lightly flour your work surface and both sides of the dough. Using a rolling pin, roll into a large thin circle. Place in a deep pie pan.
Optional: sprinkle the thin semolina on the bottom of the pastry dough. This will help to absorb the juices of the filling and avoid for the bottom of the pie to be soaked.
Pour the Chicken and Mushroom Filling over the Pastry in the pie pan.
Roll the second piece of pastry dough and place over the pie filling. Gently fold the first pastry dough over the edges of the pastry lid to seal the pie. Use any leftover pastry to decorate your pie if desired. With a sharp knife, cut a small cross in the middle of the pastry to allow for any steam to come out of the pie while it bakes.
Whisk the Egg and brush it over the pie pastry.Bake for 30 to 35 minutes or until the pastry is golden-brown.Leave to cool down for 10 to 15 minutes before serving.
To make without a food processor, either use a pastry cutter or your hands to cut the butter into the flour. Rub the butter and flour between your fingers and palms to get the crumb consistency, then add the cold water and quickly knead until you get a uniform dough.