Preparation: thinly slice the onions, peel and chop the tomatoes, : peel and thinly slice the potatoes Time: 30 minutes preparation, 1 hour chilling, 25 minutes cooking time. Place the butter, iced water and lemon juice into a large bowl. Add quarter of the flour and mix to form a paste using a fork. Add the remaining flour and mix to form a smooth dough. Wrap in plastic wrap and put in the fridge for one hour. To prepare the filling heat the oil in a large pan. Add the onions and cook, stirring over a low heat until they are a golden brown. Add the tomatoes, sugar and vinegar. Stir to combine. Simmer, uncovered, until thickened. Heat the extra oil in a large, shallow pan. Add the potatoes in a single layer. Cook, turning once, until the potatoes are golden brown. Roll out the pastry between two sheets of greaseproof paper to fit a 23cm fluted flan tin. Line the tin with the pastry and bake for 25-30 minutes or until crisp and golden brown. Spread the onion filling over the base of the cooked pastry. Arrange the potatoes on top of the onion. Spread over the sour cream and sprinkle with paprika. Bake in a hot over (200C) for 8 minutes or until the top is golden brown. Serve hot or at room temperature.