Heat the oil in a wok or large frying pan over a medium heat. When hot put in the mustard seeds, urad dal and chillies. Stir until the mustard seeds pop.
Add the curry leaves and stir once or twice. Add the onions and stir and fry until they begin to brown at the edges. Add the salt, turmeric and cayenne and stir for 5 seconds.
Add the potatoes and stir and fry for one minute. Add the tomato and the asefetida. Stir and fry for another minute then add 1 cup plus 2 tbsps water and bring to a boil.
Cover, turn the heat to medium and cook fairly vigorously for about 10 minutes or until the potatoes are just tender and the water is absorbed.
Remove the lid and stir and fry the potatoes on a medium low heat for 4-5 minutes to get a rich colour.