Saute and salt the onions. Grind the potato and add it to the onions. Add the peeled tomatoes, thyme, oregano, salt, pepper, low fat butter, and walnuts.
Lay the three pastry sheets radially in a large, shallow oiled baking pan, sprinkling them with oil in between. Empty the stuffing in a thin layer all over the sheet surface. Twist the sheet edges inwards over the stuffing.
Cover the pie with the remaining pastry sheets, laying them in pleats, after sprinkling them with oil in between.
Curve the pie, sprinkle the surface with a little olive oil and water using your fingers. Bake for 1 hour at 180°C, with extra heat on the bottom rack. Take the pie out of the oven, let it stand for 15 min and cut it in pieces.