Can I freeze caramelised onions? Caramelised onions seem easier to make in large batches, so could I make a big batch then freeze them in say, ice cube trays, for later use?  Or would this have a detrimental effect on them?
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.