In a large bowl, combine flour with the salt and butter and mix with hands till crumbly. Add water and blend just until dough comes together.
Shape into a disc and put in the fridge.
Prepare the filling:
Cook potatoes in a pot of water until tender. Keep 1/2 cup of potato water and drain the rest. Mash potatoes and set aside.
In a frying pan, cook the onion, garlic beef and pork over medium heat until no longer pink. Drain off excess fat.
Combine the seasonings in a small bowl and add to the meat mixture in the skillet.
Add potato water. Mix in well and simmer over low heat for about 10 minutes until the liquid is absorbed.
Remove the pan from heat, stir in mashed potatoes, and set aside to cool.
Preheat oven to 200°C
Make the meat pie:
On a lightly floured surface, divide dough in half and flatten one ball of dough with your hands. Roll dough to about 12-inches in diameter to fit a 9-inch pie pan.
Place the pastry into the pie plate and add the meat filling.
Brush around the outer edge of the pastry with the beaten egg. Roll out the top pastry and place on top of the filling.
Fold the top crust under the bottom crust and pinch the edges. Brush with egg wash and cut vent holes.
Bake in a preheated oven for 30 to 35 minutes or until the pastry is golden brown.
Remove the tourtière from the oven and let it cool at least 10 minutes before serving.