Preheat the oven to 200C/400F/Gas 6.
Place the chicken, peppercorns, and bay leaf in a pot on the stovetop. Pour in the mixture of milk and chicken stock. Bring mixture to a boil and let it simmer for 25 minutes or until all the chicken pieces are thoroughly cooked.
Meanwhile, fry the ham or beef’s tongue in the butter. Add the sliced leeks, celery, onion, parsley, and thyme, and simmer until they soften.
Once the chicken is fully cooked, use tongs or a spoon to remove it from the milk. Use a fork to pull it apart into strips/flakes. Discard the bones, if any, depending on the cuts of meat chosen. Now add the pulled chicken meat back into the pan with the ham or beef’s tongue and leeks.
To the mixture of meat and leeks, add the flour. Fry for two minutes until well combined.
Pour the milk into the pan through a sieve to remove the bay leaf and peppercorns.
Add the dried mustard. Stir the mixture constantly until it thickens. Turn off the heat.
Roll out the puff pastry into rectangular sheets about 3mm thick.
Spread the filling over one rolled dough sheet. Cover it with another dough sheet and crimp the edges with your fingers.
Beat one egg in a bowl.
Glaze the top of the pie with the egg by brushing it on. Sprinkle the grated Parmesan across the top to taste.
Bake the pies on lightly greased baking racks for 35-45 minutes.