Preheat oven to 230C/450F. Cut eggplant in half lengthwise and place on an ungreased baking sheet with the cut side down. Bake for 30 to 40 minutes or until the eggplant wrinkles and feels soft to the touch when pressed. The timing will vary depending on the thickness of the eggplant.
Allow the eggplant to cool. Discard the skin, finely chop the flesh and mash it with a fork.
Heat oil in a skillet over medium low heat. Fry kalonji seeds and whole chillies for a few seconds. Add onion and fry until it is richly browned but not burnt, 8-10 minutes. Add garlic and ginger and stir several times. Add chopped green chilli, turmeric, salt, sugar and tomatoes. Simmer, covered, until tomatoes have disintegrated into a sauce, about 10 minutes. Discard whole chillies, if desired. Add the eggplant and simmer, covered, 10 minutes to blend flavours, stirring occasionally to prevent sticking. Remove from heat. Let stand covered for 15 minutes to help develop the flavours.
Add coriander and mix well. Garnish with green onion and serve.