The dough can be made in advance. You will need 1 pound of dough to make this pie. Blend the shortening, flour and cold water together in the bowl of a stand mixer on low speed, using a dough hook. Mix until well blended.
Remove the dough from the bowl, wrap in plastic wrap and put it on a sheet pan. The dough can be stored for up to 1 week in the refrigerator, but allow it to come to room temperature before rolling.
Cut the dough in half and roll out 1 piece of dough to fit into a 10-inch pie plate. Fit the dough circle into the bottom of the pie pan, pushing it snugly against the edges. Roll out the other piece of dough and set aside until ready to use.
Preheat the oven to 350 degrees F.
Bring a medium pot of salted water to a boil over medium heat. Add the potatoes and blanch until al dente. Drain and set aside to cool.
In a large skillet over medium heat, add the beef, pork, and the onions.
Brown the meat, then discard the grease. Add the potatoes and moisten the mixture with the beef soup base. Add instant potato powder along with cinnamon, nutmeg, garlic, black pepper and 1/4 teaspoon salt. Mix well.
Pour the filling into the dough lined pie plate. Level it off and cover with the remaining circle of dough. Pinch the edges of the bottom layer and top layer of dough together and create the knuckle-like pie edge ridges. Trim off any excess dough using a sharp knife.
Brush the top of the pie with the egg wash. Make a few slits in the top crust to allow the steam to escape.
Bake until cooked through and the top of the pie is golden brown, about 45 minutes. Remove from the oven and serve.