Soak the aubergine in cold water for 10 minutes. Drain and pat dry with kitchen paper.
In a large frying pan, heat 4 tablespoons of the oil over a medium heat. Add the aubergine and a little salt, and cook for 10 minutes or until the aubergine is brown and soft. Drain the aubergine on kitchen paper, and set aside in a warm place.
Heat the remaining oil in the same pan. Add the mustard seeds and, as they begin to pop, add the curry leaves and urad dal. Cook, stirring, for a few minutes or until the dal is golden, then add the onions and a little salt and cook until the onions are starting to colour, stirring occasionally.
Add the tomatoes, tomato purée, chilli powder, coriander and turmeric, and mix well. Add the aubergine and cook for a further 5 minutes, stirring occasionally, until the tomatoes break down. Transfer the mixture to a serving dish, and serve immediately