To prepare the chicken pot pie with pie crust start heating together the oil and butter in a casserole dish set over a medium heat until hot.
Add the carrots, onion, and a little salt, sweating for 6-7 minutes until softened. Add the chicken, stir well, and cook for a further 4-5 minutes until starting to colour.
Cover the mixture with the chicken stock and cook until simmering. Cook steadily for 10 minutes until slightly thickened.
Whisk together the cornflour and milk to make a paste. Whisk the paste into the gravy to thicken, making sure the gravy is simmering.
Stir through the peas and parsley. Season to taste with salt and pepper.
Preheat the oven to 170°C (150°C fan). Spoon the filling into a 900 g round pie dish.
Roll out the pastry on a lightly floured surface into a round approximately 30 cm x 1/2 cm in dimension.
Drape the pastry over the filling, sealing it against the inside of the rim of the pie dish. Brush with the egg yolk and sprinkle over the sesame seeds.
Bake the chicken pot pie with pie crust for 1 hour 5-10 minutes until golden and puffed before serving.