Preheat the oven to 190°C.
Roll out three quarters of the pastry to 0.5cm thickness and set the rest aside.
Line a 23cm springform tin with pastry, add parchment and baking beans.
Blind bake for 15 minutes, remove beans, then bake for 5-8 minutes until golden.
Sprinkle semolina flour over the bottom of the baked pastry case.
Boil sliced potatoes for 3 minutes, add onions, and simmer until tender.
Drain vegetables and mix with double cream, flour, salt, and pepper.
Layer apples, then potato mixture, sage, ham, and sugar in three layers.
Slowly pour the cider over the filling ingredients.
Roll out the remaining pastry to create a lid for the pie.
Brush edges with egg, seal the lid, and cut a steam vent in the center.
Bake for 60 minutes until golden and bubbling, then cool before serving.