Make one small slit with a knife (this makes sure they will not burst and send the content all over you grill or oven),
Place on the grill or on a cookie (lined with foil or a silicon mat) in the oven. on the grill, you need to check it often and turn them as you see that it getting charred. In the oven, check after 10-15 minutes and turn as they get charred. Check after 7-8 after the first turn.
It may take about 30 minutes
Once done, remove from grill or oven, let cool till you can handle it.

Make one vertical cut and scoop out the flesh with a tablespoon.
If using fresh - transfer the eggplants to a strainer for a couple of minutes to allow the juices to drain.

Finely chop the eggplants with a knife until they form a smooth puree. (This traditionally better done with a wooden spoon if available). Add the eggplant puree to a bowl and stir in the olive oil and lemon juice until well-blended; taste and add more olive oil and lemon juice if desired. Add the onion and mix in the mayonnaise, adding more if needed. Season with salt and pepper. Garnish with dill and refrigerate for at least one hour to chill.

If prepared for freezing - no need to chop, just place the flesh of one eggplant in a sandwich size zip lock and freeze. Do not add any other ingredients.
Lasts practically for ever in the freezer. Defrost when want to use.