how do I crisp onions that have been frozen?

You don't. That is why you can't freeze onions. Same goes for cucumber, noodles etc.

I would only use frozen onions in cooked food. If you do that you can saute or fry the onions in a little butter or oil as usual.

frozen onions, is a bad deal, about all they are good for is flavoring in soups and the like.

There is only three ways, slice and dehydrate until crisp, slice and refreeze with liquid nitirogen until crisp, slice and dredge in seasoned flour and deep fry until crisp.