Combine the pork, pepper and water in a large saucepan or skillet—breaking up the pork into small pieces. Bring to a boil, then reduce the heat to medium-low and add the spices and cornstarch. Cook, covered, for 30 minutes or until the water has evaporated.
Meanwhile, sauté the onion, celery and salt in olive oil until translucent. Add the garlic and butter and cook for 5 more minutes. Combine with the pork mixture.
Preheat the oven to 425°. Prepare the bottom piecrust and pour in the filling. Seal with the top crust and prick, or make slits, for ventilation. Bake for 10 minutes, reduce the heat to 350° and bake for 40 more minutes. Serve hot with homemade mustard.