Cut each clove of garlic in half. Make 6 small slits in the baingan and stuff the slits with the halved garlic cloves. Roast on an open flame for 8-10 minutes, turning the baingan every 1-2 minutes to make sure its roasted well on all sides. The skin should be blistered completely and the baingan should become really soft. You can prick it with a fork or knife to check if its cooked from inside. If not, roast for a few more minutes.
Once the baingan is roasted, cover it with foil and set aside to cool for a few minutes. Peel the skin off and submerge it in water to get rid of any extra burnt bits. Remove it from the water, cut off the stem and chop it finely.
Heat oil in a pan and add nigella seeds. After 30 seconds, add garlic, ginger, chopped chillies and onions. Stir fry till the onions are soft and add tomatoes, chilli powder, coriander powder, turmeric and salt. Mix well, and cover and cook till the tomatoes are soft and pulpy. Add the chopped baingan and mix well. Cover and cook for another 3-4 minutes while stirring occasionally. Add chopped coriander, mix and serve hot.