Saute the ground beef and onions lighly in butter or olive oil. Remove from heat and add remaining ingredients except pastry sheets and olive oil. Mix well.
Oil a 9x13 inch baking pan and line with 6 pastry sheets, brushing each with oil. The pastry sheets must come up the sides of the pan. Spread the meat filling evenly and cover with remaining pastry sheets, brushing each sheet with oil. Fold the edges under to seal in the filling. Score with a sharp knife the three top pastry sheets in 3 inch strips from one end of the baking pan to the other. Brush the top with remaining oil and sprinkle with water. Bake pie in moderate oven for about 45 minutes. Cut into squares and serve hot.