Heat the grill. Slice the aubergines lengthways into 1.5cm (¾in) strips. Put under the grill and cook until golden, then turn and repeat. Don't take your eye off them as they brown very rapidly.
When cooked, put in a lidded plastic container and leave to cool, then chop into small pieces. Dip the tomatoes in a pan of simmering water for 30 seconds, remove and allow to cool. Then peel away the skin, cut each tomato into quarters, remove all the seeds and dice into small pieces.
Mix onions and all of the above with the olive oil. Dry fry the cumin for 2-3 minutes and add to the aubergine, with the marjoram, and mix well.
The salsa can be made a day in advance - store it in the fridge but serve at room temperature.