My friend (not vegan) has exactly that problem. Once she had a bite of some pasta dish ith hidden onions in it and was gagging for ages even though she didn't know there were onions till after she'd eaten some! She's always been that way though, and I guess some of it might be psychological.
From what I know, food intolerances are built up by excessive amounts of that food or certain chemicals in it, so maybe you're body literally got sick of onions from when you loved them? Thing is, though that after six weeks exclusion you should eb able to incoporate them back in to your diet without any problems, and if it's been ten years, you must have it pretty badly!
Maybe you should see a holistic nutritionist if you're in to that kind of thing, or a food intolerance specialist such as at Holland and Barret ect? Good luck.
A computer can beat me at chess, it can confuse me beyond belief and it can play mind games with me but it is no match for me when it comes to kick boxing.