Preheat oven to 200°C (390ºF). Heat a deep-sided 2 litre-capacity, 20cm frying pan over medium heat. Add the butter, leek and garlic and cook for 4–5 minutes or until the leek is soft.
Add the chicken, breaking any lumps with a wooden spoon, and cook for 6–8 minutes.
Stir in the flour, add the stock, salt and pepper and cook for 3–4 minutes or until thickened. Stir through the parsley. Remove from the heat and set aside.
Roll out the pastry on a lightly floured surface until 3mm thick. Use a sharp knife to cut out a 24cm round and a small hole in the centre of the round. Place on top of the pie and press the edges to seal. Brush with egg and bake for 20–25 minutes or until the pastry is puffed and golden. .