Combine flour with salt. Cut in butter and shortening until mixture is a roughly even crumbly texture. Add lemon juice and water and blend just until dough comes together.
Shape into a disc, wrap and chill for 30 minutes. While pastry is chilling, prepare filling.
Cook 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 beef, pork and/or veal over medium heat until no longer pink. Drain off excess fat, add onions and seasonings and sauté until tender, about 10 minutes.
Add cider and water and bring up to a simmer. Let mixture simmer for about 15 minutes, until most of liquid is absorbed. Remove from heat, stir in potatoes and cool to room temperature. This can be prepared a day in advance.
Preheat oven to 375 °F.
On a lightly floured surface, cut dough in half, roll out to just less than ¼-inch (.5 cm) thickness and line an 8-inch (20 cm) springform pan. Fill with tourtière filling.
Roll out remaining dough, cut a hole in center (for steam to escape) and place on top of filling. Pinch edges of crust together and brush with eggwash. Bake for 40 to 45 minutes, until pastry is a rich golden brown. Let cool 5 minutes, then remove from pan and serve.
Tourtière can be made up to 2 days in advance and reheated in a 300 °F oven.
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