Lightly prick the eggplant to prevent them from bursting while in the oven.
Place eggplant on a baking sheet lined with foil and roast for about 45 minutes or until the middle feels soft while the peel is very crisp (chances are the peel will crack, exposing the flesh which will make it easier for you to check the "doneness" of the flesh)
Leave to cool until you're able to touch the eggplant without burning your fingers.
Once cooled, peel roasted eggplant then place them into a strainer
Press out as much of the juice as possible.
Finely smash with a knife until you get a paste like, smooth mixture.
Strain again. Make sure all water is out before starting to prepare the dip.
Place the mashed eggplant in a large bowl, add salt and lemon juice and mix until well incorporated
Start adding oil, a few drops at a time and mix well to incorporate. Only then continue adding oil, gradually.
Add mustard and mix to combine.
Finally add finely chopped onion and mix well to combine
Serve cold with crackers or pita chips as a dip or spread on bread or crostini.