Over an open heat, roast the entire eggplant until the skin is soft and browned.
Peel the eggplant and remove all of the burned skin after it has cooled.
Remove the large seeds from the eggplant and mash it until it's smooth.
Chop the fresh coriander leaves, green chiles, and onions finely.
Add a pinch of salt, chopped onions, coriander, and chilies.
Pour in the mustard oil and stir until everything is nicely mixed.
Serve with fresh roti or hot litti for a traditional Bihari meal.