Heat the oil in a small saucepan, add the curry leaves and mustard seeds, then cover and cool, off the heat, for a couple of minutes. Add the onion, chillies and turmeric and saute for 5 mins.
Meanwhile, dice the potatoes. Add them to the pan, toss in the salt and pour in just enough water to cover. Simmer gently until the potatoes are tender. This masala should be fairly dry and medium-hot to taste.