Cook the eggplant: You can either grill the eggplants or roast them in the oven at 375 degrees. Place them on a baking sheet, poke some holes in the eggplants with a fork, this is a very important step. If you don't poke them, you'll risk them exploding in the oven. Make sure they are baked really well, for about 45 minutes, turning them every 15 minutes using a pair of tongs. Once they are done, let them cool completely.
Peel the eggplants: Once cooled start peeling them using a knife. If you've cooked them enough, the peel should come off easily. Make sure to remove any of the dark peel. Transfer them to a colander over a bowl and let them drain for about 20 minutes. You want to extract as much water out of them as possible.
Blend the dip: Place the eggplants in a food processor, add the onion, salt and oil and pulse a few times until smooth. Chill for about 30 minutes before serving, it's best served cold.
You can store this dip in an airtight container in the fridge for up to 1 week. This recipe also freezes well too, freeze for up to 3 months and allow to thaw completely before feasting.