Heat the oil in a large, non-stick, lidded wok or frying pan and fry the potatoes for 10 minutes until really golden all over. Add the onions and fry for 5 minutes until soft and golden. Tip in the purée along with lots of seasoning, the ground turmeric, yogurt and 300ml of water. Mix well, put on the lid and simmer for 20 minutes or until the potatoes are just tender. Remove the lid and simmer for another 10-15 minutes or until the sauce has thickened. Season and sprinkle with coriander to serve.