If making homemade pie crust: Prepare dough the night before, or at least 2 hours prior, in order to adequately chill the pie crust dough. If using store-bought pie crust: Allow frozen pie crust to thaw on the counter for approx. 4 hours, or per manufacturers instructions.
Begin preparing the filling: In a medium sauce pan, combine potatoes, water and salt, and bring to a boil. Boil until potatoes are easily pierced with a fork, about 6 minutes.
Drain potatoes and reserve the potato water. Mash half of the potatoes, set aside.
In a large skillet, cook ground meat over medium heat until browned. Break into small pieces with spatula as you cook. Drain off excess fat.
Combine the seasonings in a small bowl. Add the seasonings, onion, and reserved potato water to the meat in the skillet. Mix well and simmer over low heat for 20 minutes, or until the liquid has evaporated and the onions are tender.
Remove the pan from heat. Add the mashed and diced potatoes to the meat mixture, gently stirring until thoroughly combined. Set aside to cool.
Preheat oven to 375°F/190°C.
Arrange the meat pie. If using homemade pie crust dough: On a lightly floured pastry mat or clean work surface, roll dough to about 12 inches in diameter to fit a 9-inch deep dish pie pan (at least 1 and 1/2 inches deep).
Gently place the dough into the pie pan and trim excess dough with a sharp paring knife around the edges.
Spoon the meat mixture into the crust, gently patting it flat.
Combine the egg with 1 T of water and brush the egg wash on the outer edges of the pie dough with a pastry brush.
Roll the other piece of dough into a 10 inch circle and place on top of the filling. Tuck the overhanging bottom crust over the edge of the top crust, pinching to seal with a fork or crimp with fingers. Brush the top of the pie with more egg wash and cut a vent hole with a sharp paring knife in the top of the pie. Make it decorative if you choose!
Bake prepared pie in preheated oven for 35-40 minutes, or until the pie crust is golden brown.
Serve with ketchup and pickles for an authentic experience!