Caramelized onions?

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oooh good choice on the red onions!! :)

You dont need the balsamic to caramelise the onions though. You can use some brown sugar, treacle is really nice too actually.
Cook them down quite a way first, then add the vinegar/sugar/ etc and let them sit for little while - you want them to brown (careful you dont burn them though! lol)

Any other vinegar probably wont work as it isnt as sticky as Balsamic, but you could try white balsamic or Kechap Manis ( a very think sweet indonesian soy sauce.)

Anyway, a bit of brown sugar works really well and is yummo with the red onions! :)
Let us know how you go

Yeh, like Alexis i was also going to suggest brown sugar ;) ... Bet you are either cooking or eating them right now, LOL !!