In a large frying pan, add oil and pork. Cook on medium-high heat until the pork is cooked through, then add onion and spices, continuing to cook until the onion turns translucent.
Lower heat to medium-low, add the water and simmer to deglaze the pan.
Once the caramelization is removed from the bottom of the pan, add the mashed potato, a little at a time, and simmer the mixture until it is nice and thick.
Remove from heat and let the filling cool before pouring it into the pie crust.
When the tourtière is filled, roll out the top crust, making sure to crimp the crust to seal the edges.
Brush with egg wash, if desired.
Cut a hole in the center of the crust so that steam can vent, then bake at 425º F in the center of the oven for 40 minutes, or until the crust is golden brown. Cover edges with foil if they're browning too quickly. Makes 1 pie that serves 6-8.
Serve hot or cold.These pies also freeze well. Let thaw completely before baking.