Heat the oil in a wok over medium heat and cook the mustard seeds until they pop, about 1 minute. Add the cumin seeds and gently stir-fry for 1 minute. Add the onion and garlic, and stir-fry until light golden brown, about 5 minutes.
Add the eggplants, chilli powder, coriander, turmeric and salt, and stir-fry for 1 minute. Then add the tamarind juice, reduce th