Cut off 1/3 of the dough to use for the top crust. Wrap this portion in plastic wrap to keep it warm and moist, and set it aside.
On a floured counter, roll out 2/3 of the dough into a circle large enough to fit into your pan with an inch or two hanging over the sides.
Fit the dough into the pan, pressing it against the sides.
Spoon the prepared meat filling into the crust, then top the meat filling with the mashed potatoes.
On a floured counter, roll out the remaining 1/3 of the dough, then place the top crust over the filling.
Use scissors to trim the edges, leaving enough overhang to press the top and bottom crust together. Use your fingers to crimp it all around.
Gather up any remaining dough scraps, roll them out again (the dough may be stiffer and more crumbly now), and cut out leaves or other decorative shapes.
For the egg wash, whisk together the egg with 1 tbsp water. Brush the egg wash over the top crust. Place the decorative cutouts on top, and brush those with egg wash, too.
Use a knife to poke a hole in the center to vent steam.