Rub the eggplant with little bit of oil, make slits in them so while roasting them steam gets to escape, and grill them over an open gas flame or outdoor grill until the skin is charred and eggplant is cooked through. Keep turning the sides of the eggplant using tongs until it is charred from all sides.
Set the eggplant aside to cool while you chop the onions and tomatoes.
Once it has completed cooling off, peel the charred skin off the eggplant. I remove the seeds also as I am not a fan of the seeds. You can either leave the seeds or remove them. Roughly mash the eggplant with the back of your fork.