Preheat the oven to 200°C (400°F). Lightly grease a 20cm / 8in pie tin.
Make the hot-water crust pastry: put the Plain Flour and a pinch of Fine Salt in a large bowl. In a small saucepan, gently heat the Lard with Water until the lard has melted and the mixture is just coming to a simmer.
Pour the hot fat and water into the flour, stirring with a wooden spoon until a soft dough forms. When cool enough to handle, knead briefly on a lightly floured surface until smooth. Reserve about a quarter of the dough for the lid and shape the remainder to fit your tin.
Line the prepared tin with the larger portion of pastry, pressing it up the sides and forming a well for the filling. Trim any excess pastry around the rim.
Prepare the filling: coarsely chop or roughly mince equal parts Pork Shoulder and Pork Belly. Place in a bowl and season well with Fine Sea Salt, Ground White Pepper, a generous pinch of Ground Mace and a little grated Nutmeg. Mix thoroughly so the spices are evenly distributed.
Pack the seasoned pork mixture firmly into the pastry case. Roll out the reserved pastry and cover the pie, crimping the edges to seal. Brush the top with beaten Egg and cut a small hole or make a removable plug in the centre to allow steam to escape and to pour in the jelly later.
Bake on the middle shelf for 30–40 minutes until the pastry is deep golden and the filling is cooked through (internal meat temperature should reach at least 70°C / 160°F). If the crust browns too quickly, cover loosely with foil.
While the pie bakes, make the jelly: warm the Pork Stock gently — do not boil. Soak the Gelatine Leaves in cold water for a few minutes, squeeze out excess water and whisk them into the warm stock until fully dissolved. Taste and adjust seasoning if needed, then keep the jelly warm but not hot.
When the pie is baked, allow it to cool for 20–30 minutes. If you made a central plug, remove it and pour the warm aspic into the pie through the hole until the cavity is filled. Replace the plug or seal the hole with a small piece of pastry if desired.
Cool the pie to room temperature, then refrigerate for several hours or overnight so the jelly fully sets. Serve chilled or at room temperature, sliced thickly.
