Smoke or roast the eggplant. Hold the eggplant over a low open gas flame. Rotate frequently.
In the meantime, keep all your other ingredients ready.
After about 20 minutes, when the skin is completely charred, remove the eggplant from the flame.
Allow the eggplant to cool, then discard the skin. Mash the eggplant with the back of a fork or use your hands.
Add the onion, green chilies, coriander leaves, mustard oil and salt and mix well. The smoke-kissed Bengali Begun Pora is now ready to be eaten with ruti.