Can I freeze caramelised onions?
I would say it depends. I find once you freeze an item the flavor/texture/propeties of the item is changed (for better or for worse) the next time you goto use it.

If your using caramelized onions in a soup, or stew, something where flavors blend together for a longer period of time, then yes go ahead.

however if your using the caramelized onions on top of a steak or something similar. Then my answer would be no.