Heat the 2 tablespoons oil over medium-high heat in a large frypan and fry eggplant pieces until golden, about 5 minutes. Remove the eggplant from the frypan and set aside; wipe out the frypan with a paper towel.
Heat the 6 tablespoons oil in the same frypan over medium heat. Stir in the cumin, fenugreek, kalonji and sesame seeds and cook until the cumin turns golden, about 2 minutes. Reduce the heat; stir in the ginger and garlic and cook for a few seconds. Stir in the onions and green chilli and continue to cook and stir until the onion is golden, about 10 minutes.
Stir the tomato puree, chilli powder, coriander, turmeric and salt into the onions. Cook and stir over medium heat for 2 minutes or until the oil separates. Stir in the cooked eggplant; cover and simmer until eggplant is tender, about 5 minutes. Pour in the coconut milk; stir until blended and heated through. Sprinkle with coriander leaves.
Kalonji (nigella seeds) are also called onion seeds because of their flavour and are found in Asian supermarkets in small packets. These small, pointy black seeds are from a flower related to the garden flower love-in-a-mist.