In a small saucepan, on medium heat, melt the butter and add the flour.
Slowly add the chicken stock while stirring constantly, till the mixture becomes like a semi-runny paste. Turn off the heat and set aside.
For the Filling
In a medium saucepan, add olive oil and leeks and cook till softened.
Stir in the sliced mushrooms, minced garlic, sage, thyme, parsley and salt and stir. Cook till mushrooms are softened.
Add the pieces of chicken and 1 cup of the chicken stock. Simmer for 5-7 minutes to reduce. There should be a couple of tablespoons of liquid remaining.
Turn off the heat and add the roux to thicken it, mixing well.
Make the Pie
Preheat oven to 390℉/200℃. Press down the pastry to line a non-stick baking dish so the pastry come up and lines the sides to reach the rim.
Cover the pastry with some parchment paper, weighed down with baking beans and bake for 15 minutes.
Remove from the oven and carefully take out the parchment paper and beans, then bake for a further 5 minutes or till golden. Remove from the oven and allow to cool before spooning out the filling onto the pastry.
Cover the filling with another layer of pastry, pressing down on the edges to seal the sides. Brush with milk or egg white.
Repeat for the 3 other pies. Bake for 20 minutes until the pastry has risen and is golden brown. Allow to cool slightly before serving.