First, cook the pork belly. Season the belly, add all the remaining ingredients to a roasting tin then nestle the belly on top, cover with cling film and foil and slow roast in a preheated oven at 150 degrees for 3 hours or until tender.
Once done allow to cool and pick the meat into strands, discard the fat and set the meat aside.
Sieve the braising liquid and reduce until approximately 500ml remains. Set aside. Remove the roasted garlic and squeeze the softened cloves from the head into a small bowl and stir to create a puree. Set aside.
To make the pastry in the food processor: blitz half the lard and flour together until there are no lumps. Add the remaining lard and continue to mix until the mixture looks like breadcrumbs. Add the cold water gradually and keep mixing until you start to get a dough. Remove and wrap tightly in clingfilm and chill in the fridge for at least an hour, preferably overnight.
To make the filling: melt 50g of the butter in a large frying pan over a high heat. When the butter is foaming, add the mushrooms, season, and fry quickly until coloured. Set aside in a bowl.
Without cleaning the pan, drizzle in a little oil. Add the onion, season, and sweat gently for about 15 minutes, stirring from time to time, until soft. Remove the onions from the pan and add to the bowl of mushrooms. Then add the bacon to the pan that you cooked the onions in and fry until the fat has rendered, and is starting to brown. Add the leeks to the bacon, season, and sauté until soft or for approximately 5 minutes. Remove the mixture from the pan and add to the onions and mushrooms.
In a saucepan over a medium heat, melt the remaining 30g of butter. Add the flour and cook, stirring continuously for a couple of minutes. Gradually add the cider reduction, followed by the milk and finally the cream, continuing to stir until thickened. Cook for a further 4–5 minutes, stirring, then add the mustard. Check for seasoning and turn off the heat and leave to cool for a few minutes. If you’re making a pouring sauce, remove three heaped tablespoons of the mixture and add milk to loosen to a sauce consistency.
Add the pork belly, followed by the herbs and roasted garlic to the mushroom, bacon, leek and onion mixture, then stir the mixture into the sauce. Transfer the mixture to a bowl, cover with cling film and chill in the fridge for at least an hour.
Preheat the oven to 180°C. Roll out the pastry and cut into circles big enough to line a 6 cup, 7cm muffin tin. Depending on the depth of the tin, you may have some pastry left over. Using kitchen towel, rub lard around the muffin cups, and push the rolled-out pastry into the dish, trimming any excess left at the top. Spoon in the chilled pie filling, pushing it right into the edges. Egg wash the top of the pastry.
Roll out the remaining pastry into 8cm circles large enough to form the lid, then drape it over the top of the pie, pushing down around the edge with your finger to form an initial seal, then follow with a small fork to leave a neat print around the edge and finish the seal.
To glaze the pies: mix together the eggs and milk and brush onto the top of the pie. Sprinkle over the flaky, smoked sea salt.
Place the tin into the preheated oven and cook for 45–50 minutes until the pastry is crisp and golden. Remove from the oven and leave to rest for 10–15 minutes before turning out and serving.