Heat the oil in a large saucepan over a medium heat. Add the green chillies, ginger, curry leaves, mustard seeds, turmeric and chilli powder and cook, stirring for 1 minute. Add the onion and continue cooking for 4-5 minutes or until it is lightly browned. Stir in the green beans, tomatoes and 4 t water. Cover and cook for 10-15 minutes over a low heat, stirring occasionally, until the beans are soft and tender. Add the coconut, then some salt and pepper to taste and mix well before serving.