Preheat the oven to 350˚F.
In a large skillet set over medium-high heat, heat a drizzle of oil and sauté the onion and celery for 3-4 minutes, until soft.
Add the ground meat, garlic, parsley, nutmeg, cinnamon, salt and pepper.
Add a half cup of water and simmer for half an hour. Stir in the potato.
On a lightly floured surface, roll out half the pastry into a 10-inch circle and line a pie plate.
Fill with the meat mixture, roll out the remaining pastry and lay it overtop.
Trim and crimp the edges, and cut a few slits in the top to help the steam escape.
Brush the pie with beaten egg. Bake for 45 minutes to 1 hour, or until golden.