Dust the aubergine slices with the turmeric, salt and half the red chilli powder.
Heat the oil in a large, non-stick frying pan and fry the aubergine pieces until soft (you may have to do this in two batches). Set aside.
In a bowl, beat 200ml/7fl oz of the yoghurt with the sugar, salt and the remaining red chilli powder. Pour into a small saucepan and heat gently, stirring often to stop it from splitting, for about five minutes.
Stir in the ground cumin and the aubergine slices. Heat gently and stir to combine.
Stir in the remaining yoghurt, check the seasoning and serve garnished with the fresh coriander.