Never a huge fan of onions to begin with, but lately seem to be experiencing some mild allergic reaction to raw or cooked onions. Found a great recipe that calls for them and would like to sub if possible.
How about caramelized shallots? Those are deliciously sweet.
If you're allergic to onions, you may be allergic to all members of the allium family--that includes shallots, garlic, chives and leeks. The wonderfulness of caramelized onions comes from the combination of the pungent onion flavor mellowed by sweetness, and there's some umami in there as well. You could try caramelizing other vegetables--carrots, celery, fennel--you may have to sprinkle a little brown sugar over them to get them to caramelize, but that might give you the flavor you're looking for.
Also, onion allergies are rare, but all food allergies should be taken seriously because they can be fatal. Consider seeing an allergist to test if you are truly allergic.
It might be easier to suggest an alternative if you specified the recipe you're making.
If you have a true allergy or intolerance for onions - as opposed to just a distaste for them - I'd imagine other members of that family (shallots, etc.) might cause the same reaction. I can hardly think of anything more delicious than caramelized onions or shallots, so I hope that's not the case!
Or, what drbabs said. Our answers crossed in cyber-space....another reason an edit button in Hotline would be a good thing ;)
Depending on your recipe carrots or perhaps even radishes might work.