In a saucepan, heat the oil and add the onions. Saute slowly over medium to low heat until they start to caramelize and turn quite brown but do not burn.
Add the ginger and garlic and saute for another minute.
Add the coriander powder, cumin powder, chilli powder and turmeric. Mix well and stir to toast the spices.
Add the tomatoes and banana peppers and, over medium heat, cook until most of the juices of the tomato have evaporated.
Scoop out all the flesh from the cooled eggplant skins and chop roughly. Add to saucepan. Mix well.
Cook for another 5 minutes until the eggplants and tomatoes have turned into a homogenous mush. Add salt to taste.
Turn off heat and add coriander. Mix well and serve hot with naan.