Heat the oil in a large saucepan and add the mustard seeds, dried chillies and curry leaves. When the mustard seeds begin to pop add the onions and stir fry until lightly browned.
Add the coriander, garam masala, turmeric and chile powder followed by the tomatoes. Cook for 5 minutes.
Meanwhile peel the potatoes and cut into wedges or cubes. Add them to the pan and cook over a gentle heat for 5 minutes, stirring continuously.
Pour in the coconut milk and 100 ml water. Cook for 15-20 minutes until the potatoes are tender. Serve hot.