Preheat oven to 375°F.  In a large sauce pan or small stock pot, boil water and potatoes until very tender, about 15 to 20 minutes.  Drain well in a colander, set aside.

In a large sauté pan, heat oil and butter over medium-high heat.  Add onions and sauté until translucent, about 3 minutes.  Add ground beef and cook, breaking the meat into small pieces with a wooden spatula or spoon.  Sprinkle with salt and pepper.  Cook beef about 4 to 5 minutes or until beef is ¾ of the way cooked, some of the meat will have trace amounts of pink.²  Turn off heat.

Add potatoes to the ground beef.  Using a fork, finely mash the cubed potatoes into the mixture, stopping occasionally to stir with spatula.

Once potatoes and ground beef are fully incorporated the mixture will be thick but creamy.

Line a pie pan with a bottom layer pie crust and add meat and potato filling.  Top with second pie crust, seal the edges, and crimp with your preferred style.  Cut a few slits on the top of the pie crust to vent the steam and brush with milk.  Wrap the edges of the pie crust with aluminum foil to prevent over browning.

Bake at 375°F for 1 hour and 20 minutes.  After 50 minutes, remove the aluminum foil and cook until pie crust is golden brown.