Hi feret :)
I made a red onion marmalade last week-a load of chopped red onion (I used 1-it was only for me, but you could use as many as you liked!), enough balsamic vinegar to just cover the onions in the pan, and plenty sugar too. It was a bit of an experiment, but it was delicious!
I just started the onions off frying in olive oil, then after a few mins added the vinegar and sugar, leaving it all to reduce down (about 25 mins cooking in all). If it began to get a bit too sticky, I added more vinegar, and a bit more sugar to balance it.
It ended up deliciously sticky and soft, and just yummy! x