Make the hot water crust pastry: whisk flour and salt in a large bowl. Heat lard and water until melted and simmering, then pour into flour and stir quickly until a shaggy dough forms.
Knead the dough briefly until smooth and no longer sticky. Set aside 1/4 of the dough for the lid, cover both pieces with a kitchen towel, and let rest for 10 minutes while still warm.
Mix the pork filling: combine diced pork shoulder, pork belly, onion, salt, white pepper, mace, sage, and black pepper in a bowl. Mix thoroughly, then refrigerate until ready to use.
Shape the pastry: roll out the larger portion of warm dough and press it into a greased 7-inch springform pan, working it up the sides. Roll out the reserved dough for the lid and set aside.
Fill and seal: pack the chilled pork filling firmly into the lined mold, mounding slightly in the center. Brush the pastry edge with beaten egg, place the lid on top, press to seal, and crimp the edge.
Cut a small steam hole in the center of the lid, brush the top with beaten egg, and bake at 375°F for 45 minutes, then reduce to 325°F and bake for 1 more hour until internal temperature reaches 160°F.
Cool the pie in the mold for 20 minutes, then release the sides. Warm the stock gently, dissolve soaked gelatin sheets or powdered gelatin, and pour through the steam hole using a small funnel.
Chill the pie uncovered for at least 4 hours or overnight to set the jelly. Slice with a sharp knife and serve cold or at cool room temperature.