Pre-heat the oven to 200C/400F/Gas 6.
In a saucepan add the chicken, peppercorns, onion and bay leaf. Cover with milk. Bring to the boil and simmer for 25 minutes until the chicken is cooked.
Meanwhile, fry the bacon in the butter. Add the sliced leeks and allow to soften.
When the chicken is cooked remove from the milk and flake the chicken meat into the pan with the leeks, discarding the bones.
Add the flour and cook through for two minutes before adding the milk through a sieve to remove the peppercorns and bay leaf. Add the Dijon mustard. Stir constantly to make a thick sauce.
Roll out the sheets of puff pastry to a rectangular shape at a pound coin thickness. Lay the filling on one sheet and then cover with the second sheet and crimp the edges.
Brush the pie with a beaten egg and sprinkle the grated Parmesan over the top. Bake in the oven for 35-45 minutes.