For the pastry, combine the flour, sugar and salt together. Add the oil and blend in using a pastry cutter, electric beaters or a mixer fitted with the paddle attachment, until the flour looks evenly crumbly in texture.

Add the butter and cut in until rough and crumbly but small pieces of butter are still visible. Stir the water and vinegar (or lemon juice) together and add all at once to the flour mixture, mixing just until the dough comes together. Shape the dough into 2 disks, wrap and chill until firm, at least an hour.

The dough can be made up to 2 days ahead and stored chilled, or can be frozen for up to 3 months, thawing in the fridge before rolling.

Cook the potatoes in an uncovered pot of salted water until tender and drain. Roughly mash potatoes and set aside to cool.

In a large sauté pan or skillet, sauté the pork over medium heat until no longer pink. Drain off any excess fat, add the onions and seasonings and sauté until tender, about 10 minutes.

Add the cider and water and bring up to a simmer. Let mixture simmer for about 15 minutes, until most of liquid is absorbed. Remove this from the heat, stir in the potatoes and cool to room temperature. This can be prepared a day in advance.

Preheat the oven to 375°F. On a lightly floured surface, cut dough in half, roll out to just less than ¼-inch thickness and line a 9-inch springform pan. Fill this with the cooled torchière filling. Roll out the remaining dough, cut a hole in center (for steam to escape) and place on top of the filling. Pinch edges of crust together and brush with egg wash. Bake for 40 to 45 minutes, until pastry is a rich golden brown. Let cool 5 minutes, then remove from pan and serve.

The tourtière can be made up to 2 days in advance and reheated in a 300°F oven.