Sauté onion, garlic, celery and leeks in butter until soft.
Add the flour and cook over low heat, stirring continuously, for 2 mins.
Slowly stir in the chicken stock to form a thick sauce.
Fry the diced bacon until golden. Add to the sauce.
Fry the diced chicken and add to the sauce.
Season with salt and pepper and add the chopped rosemary.
Cool completely before making the pie.
Line a greased pie dish with shortcrust pastry so that the pastry is overhanging the tin slightly.
Note: you may need to piece together 2 sheets of pastry depending on the size of your pie dish.
Spoon in the chicken mixture.
Brush the overhanging pastry with a mixture of the egg yolk and milk, and lay another sheet of pastry on top to make a lid. Press the edges firmly together and trim up.
Brush the top of the pie with the remaining egg wash and bake in a preheated oven at 180°C for about 35-40 mins.
Allow to cool slightly before removing from the tin and serve with some crisp green salad.