Put Chicken Stock in a saucepan add 5 Sprigs of rosemary 5 sprigs of thyme and 5 peppercorns. Bring to a simmer, then infuse over a low heat for 5 minutes. Then strain to leave just the Stock.
Add the chicken thighs to the pan, bring back to a simmer and cook gently for 20 minutes. Add the leeks and cook for 10-15 minutes until soft.
Strain the stock into a clean bowl, and set aside the chicken and leeks to cool.
Melt the butter in a large saucepan over a medium-low heat. Add the flour and cook, stirring, for 1 minute to make a roux. Swap the spoon for a whisk. Slowly add the reserved chicken stock to the roux, whisking constantly to keep the sauce smooth. Continue cooking the sauce over a medium-low heat for 23 minutes, or until the sauce thickens.
Remove from Heat and Add Cheese and allow to melt, add the Cream and mustard.
Then Return all the Leeks, Ham and chicken to the sauce and stir to combine. Allow to Cool.
Place the flour and salt in the bowl of a stand mixer fitted with the paddle attachment. Pour in the eggs. Mix on a medium speed until well incorporated- About 30 secs.
Melt the butter, lard and 250ml water in a small saucepan over a medium heat. Turn the mixer to a low speed and gradually pour in the warm liquid, stopping as soon as a dough forms. (You can do this by hand with a large bowl and Spoon).
Spoon the cooled filling into a oven Proof dish, then roll out the pasty so bigger than the top. Place the pasty on the top and push around the edges trimming any excess.
Brush the surface evenly with beaten egg and cut a small hole in the middle of the top, to allow the steam to escape. Place in the fridge for 30 minutes (or upto a day) to allow the pastry to set.
Preheat the oven to 200C/180C Fan/Gas 6. and cook for 35-40min until pastry is golden and filling is bubbling. Take out of the Oven and rest for 5 mins before serving.
