Some people find traditionally pickled onions harsh. You suggest a proportion of balsamic vinegar Piglet.
I have also been recommended sherry vinegar, or even a medium dry sherry to be added to the final vinegar mix. Can't remember the proportions.
I suppose it depends on whether you like your onions fairly/harshly spicy, or with a bit of sweetness.
Have tended towards the harshly spicy, in the past, but would probably veer towards the sweeter in future.
valmarg