On a baking sheet add the chicken and cook for 25 min. Remove from the oven and once cooled chop into small pieces.
Preparing the dough
Divide the pastry dough in half. Roll out one half to cover the pie pan. Cover with cling film and place back into the refrigerator to cool.
Making the filling
Over medium high heat heat the oil and then add the onions, carrots and celery. Cook until softened and then add the mushrooms. Cook until the mushrooms have softened.
Add the flour and stir until all of the vegetables are evenly coated.
Add the the chicken stock, salt, pepper, spices, and chicken. Cook until the broth boils and remove from the heat. Add the peas and set aside.
Increase the oven temperature to 400°F or gas mark 6.
Assembly
Fill the pie pan with the pot pie filling. Place the other piece of dough on top and pinch the two layers of dough together. Cut air vents in the top of the crust.
Optional: To make a lattice cut to top portion of the dough into even strips. Alternate the strips horizontally then vertically alternating which strip crosses on top of the other.
Whisk the egg and add the milk in a small bowl. Brush the top of the pot pie with the egg wash and bake for 45 min. If the crust is getting dark too quickly cover the top loosely with foil.