Making the Pastry:
Start by combining 420 g of all-purpose flour with 1 tsp of salt in a large bowl. Cut in 150 g of shortening and 50 g of butter or margarine until the mixture resembles coarse crumbs. In a separate bowl, whisk together one medium egg with 65 ml of water and 1 tsp of apple cider vinegar. Gradually add this wet mixture to the flour, stirring until the dough comes together. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Preparing the Filling:
For the filling, begin by poaching 500 g of pork tenderloin in cider until fully cooked, then slice or dice it. In a pan, sauté one finely chopped medium onion until it's translucent. Add two thinly sliced medium potatoes and cook until tender. Then, mix in the cooked pork, two peeled, cored, and sliced tart apples, 150 ml of dry cider, 1 tbsp of cornflour/cornstarch, 1 tsp of chopped fresh sage (or ½ tsp dried), and salt and pepper to taste.
Assembling the Pie:
Preheat your oven to 180°C (360°F). Roll out the chilled pastry dough and line a pie dish with it, leaving enough dough for the top crust. Pour the filling into the pastry-lined dish. Roll out the remaining dough to create a top crust for the pie. Seal the edges of the crusts together and make a few slits in the top crust to allow steam to escape.
Baking the Pie:
Brush the top crust with a beaten egg for glazing. Bake the pie in the preheated oven for about 35 minutes or until the crust is golden brown and the filling is bubbling. Let it cool slightly before serving. Enjoy your homemade apple cider pork pie!