Preheat the oven to 220 degrees C. On a lightly floured surface, roll the pie pastry out into a 30cm circle. Set aside.
Melt margarine in a saucepan over medium heat. Add potato and onion and cook until the onion is tender, but not brown. Sprinkle the flour over the mixture and stir to blend. Season with oregano, black pepper and garlic powder. Pour in the beef stock and add the peas, carrot and beef. Bring to a boil. Transfer the mixture to a 2L casserole dish and place the pastry over the top. Cut slits for steam and flute the edges.
Place on a baking tray and bake for 25 to 30 minutes, or until crust is golden. Let stand 10 minutes before serving to thicken.
Fresh garlic may be substituted for the garlic powder.