Put the onion, ginger and garlic into a mini food processor and blend to a smooth paste. Cut the potatoes into small 1 cm thick slices.
Heat the oil in a wide shallow pan and add the onion paste. Fry for 10 minutes until lightly brown then add the chilli powder, coriander, turmeric and 2 tsps salt and fry for another 2-3 minutes until fragrant.
Add 100 ml water and leave the sauce to simmer, uncovered, for 5 minutes.Add another 300 ml water and the potatoes then cover and leave to simmer for 10 minutes or until the potatoes are tender.
Cut the fish into 3 cm chunks and stir into the curry. Simmer for four minutes or until just cooked through. Stir in the fresh coriander, sugar and lime juice and serve.