Halve the aubergines then cut each half into wedges. Heat 1/2 tablespoon of the oil in a large pan and brown half of the aubergines for 2 to 3 minutes on each side until golden brown and crisp all over. Repeat with remaining aubergines. Set aubergines aside.
Add the remaining oil to the pan with the mustard seeds and cook for 30 seconds. Stir in the onions and cook until softened. Add the chillies and spices with a spoonful of thick coconut milk from the top of the tin, then fry for a further minute. Add the remaining coconut milk, tomatoes and half a tin of water. Simmer for 25 to 30 minutes until thick and saucy.
Stir in the chickpeas and aubergines and continue to simmer for a further 5 minutes.
Serve with rice, warm naan bread and scatter with fresh coriander leaves.