Clean chicken well and chop into 10-12 fairly large pieces. Heat oil in a pot and stir-fry mustard seeds, onion, curry leaves, cinnamon stick, cloves, lemongrass and green chillies until fragrant. Add mixed spices and fry over low heat till oil separates.
Put in chicken to mix and stir in potatoes. Pour in water and cook covered, until chicken and potatoes are cooked through and tender.
Add coconut milk and simmer for about 2-3 minutes. Adjust salt to taste. Dish out to serve.