Cut the aubergine into round slices. For smaller ones, you can choose not to separate them at the stem. Put the ground spices in a small bowl, season with salt, and add 1 tablespoon of water. Mix together.
Coat a non-stick pan with a thin film of oil over medium heat, add the ginger paste, garlic paste and green chillies and stir-fry for about 1 minute. Add the aubergine and cook for 3-5 minutes, stirring once or twice with a wooden spatula, then remove from the pan and set aside.
Put the yoghurt in another pan and mix with a little water, then bring almost to the boil, stirring constantly to ensure that it does not curdle.
Heat the remaining oil in a heavy-based frying pan (skillet) over medium heat, add the asafoetida and mustard seeds and stir-fry for 1 minute, or until the seeds start to splutter. Add the dried red chillies, if using, and stir-fry for about 2 minutes, or until they turn a shade darker, then add the curry leaves.
Pour the tempering over the yoghurt mixture, add the aubergines and simmer for a further 2 minutes, or until the yoghurt and aubergine are hot.