MAKE THE PASTRY: Grease a 23cm (9 in.) diameter pie dish with a little butter and set aside. Stir together the flour, sugar and salt in a medium bowl, then add the butter. Rub the butter into the flour mixture until the mixture is crumbly. Pour in the egg yolk and use a fork to stir the mixture into a rough dough.

LINE THE PIE DISH: Turn the dough out onto a lightly floured surface and knead very briefly until smooth. Shape into a disk, dust with a little more flour and then roll out to a circle larger than your pie dish. Lift the pastry into the pie dish, pressing gently into the corners. Fold over the top lip of the pastry and crimp the edges. Transfer to the freezer. Heat oven to 200°C / 400°F / Gas 6 with a baking sheet in the bottom third of the oven.

SAUTÉE THE VEGETABLES: Heat the oil and butter in a large skillet or frying pan over medium heat. When the butter has melted, add the carrots and celery to the pan. Cook for 5 minutes until beginning to soften.

ADD CHICKEN: Add the leek and chicken and cook, stirring, until the chicken is no longer pink. Stir through the flour, stock powder and oregano and cook for a further 3 minutes.

DEGLAZE THE PAN: Pour the wine and milk into the pan and stir well, using a wooden spoon to scrape up any flour stuck to the bottom of the pan. Simmer for 5 minutes until thick. Season to taste with salt and pepper. Remove from the heat and stir through the parsley. If the mixture is much too thick, loosen with a splash of water.

ASSEMBLE: When the oven is hot, remove the pie shell from the freezer, and fill it with the chicken mixture. Spoon over the sour cream (if using). Top the pie with the puff pastry, then brush all over with the beaten egg wash. Sprinkle with sesame seeds.

BAKE: Bake pie in the preheated oven for 40 minutes until the top of the pie is dark golden brown and well puffed up.

SERVE: Allow to cool for 10-15 minutes before slicing and serving.