Heat the oil in a large saucepan over a medium heat. Add the mustard seeds, chillies and curry leaves. As the mustard seeds begin to pop, add the onions and sauté, stirring, until lightly browned.
Stir in the coriander, garam masala, turmeric and chilli powder. Sauté for a minute or so. Add the tomatoes and cook for 5 minutes.
Add the potatoes and cook over a gentle heat for 5 minutes, stirring constantly. Pour in the coconut milk and 125ml/4fl oz water. Cook for15–20 minutes until the potatoes are tender. Serve hot.