Place the first 4 ingredients of the Filling in a large bowl and fold gently. Set aside.
In a medium bowl place all crust ingredients and mix until a ball forms. Gently knead until the dough is smooth. Cut into 2 sections. Roll out one section at a time until it is about 1/4 inch thick. Place one crust in a prepared deep pie plate.
smooth the mashed potatoes over the bottom crust and then spoon in the the filling until it forms a heaping dome. Place top crust and seal the edges by pinching the bottom and top crusts together. with a sharp knife make 3 slits in the top crust to release steam. Scramble the egg with a tsp of water and brush the top crust until it is shiny. Place pie in a pre heated oven and bake for 35
minutes or until crust is golden brown and flaky. Let stand for 5 minutes before serving. top with extra gravy if desired.