Defrost pastry on counter until pliable, while making filling.
Heat the olive oil in a large skillet on medium-high heat. Sauté the onion, celery and carrots for about two minutes, until soft.
Add the pork, beef, garlic and other spices. Continue to cook for about five to eight minutes, or until lightly coloured.
Add the grated potatoes and water, and bring to a boil. Reduce heat to low and simmer, uncovered, for about 10 to 15 minutes, until most of the liquid has evaporated but is still juicy.
Adjust seasoning and add the Worcestershire sauce, Dijon mustard and chopped parsley. Remove from heat.
Transfer filling to a 9 or 10-inch (23 or 25-cm) pie plate or round baking dish. Cover with chilled but not frozen pastry, pressing the edges with a fork. Make slits in the top to let steam escape.
Brush with beaten egg for a golden glossy look.
Bake in a 375°F oven until pastry is golden and cooked, about 20 to 25 minutes.