Heat up the oil in a large pan.
Add the diced chicken breasts and sear them on both sides.
Remove the chicken from the pan and set it aside.
In the same pan, add the peeled and chopped onion and garlic, and fry them until golden.
Add the washed and roughly chopped leeks together with the butter, and leave them to soften for 2-3 minutes.
Pour over the chicken stock, and place a lid on the pan.
Leave it to cook for 5 minutes until the chicken is tender and some of the liquid has been absorbed.
Mix the cornflour with the milk, and add it to the pan.
Stir well until the liquid thickens into a sauce.
Season with salt and pepper, transfer the filling to an ovenproof dish, and leave it to cool down.
Meanwhile, preheat the oven to 180 degrees Celsius ( 350 Fahrenheit).
Top the pie filling with the puff pastry, and use a sharp knife to make incisions into the pastry to allow the stem to escape during baking.
Bake for 25 minutes or until golden.