Put five tablespoons of oil in a wide, deep-sided pan for which you have a lid and set it on a medium to high heat.
When hot, add the sliced red onion and a teaspoon of salt, and cook, stirring, for about 10 minutes, until the onion is soft enough to cut with a wooden spoon.
Add the garlic, cook, stirring, for two minutes, then stir in the potato wedges and oregano until well coated in oil.
Add the tomato puree, cook for a minute, then tip in the chopped tomatoes and 400 ml water.
Bring to a boil, then cover the pan, turn down the heat to simmer and leave to cook for 20 minutes.
Meanwhile, blitz the feta, yoghurt, lemon zest and juice, three tablespoons of olive oil and a big pinch or two of salt, until smooth and creamy.
Stir the olives into the tomato and potato mix, then simmer uncovered for 15-20 minutes more, until the potatoes are soft and the sauce has thickened nicely.
Serve warm in shallow bowls topped with a healthy dollop of whipped feta and a sprinkling of oregano.
