Amazing to find other people like me!
Here's my story:
I have suffered with allium intolerance since I was about 16 or 17. Like most of you I worked it out for myself - I went to my GP and got told I had "IBS" and to "just deal with it". She was very unsympathetic and gave me these fibre supplement drinks (GROSS) which made me 100 x worse. I saw another doctor when I got really bad (diahorrea and cramps every day for a month) & he referred me to a dietician who was wonderful and helped sooo much. She said taking all that fibre was probably the WORST thing I could have done!
This is what she said (in a nut shell):
Your gut is meant to contain lots of good bacteria to help with digestion.
Bacteria living on the gut wall feed off fibre.
Apparently the gas, bloating, diarrheoa etc is caused by bad bacteria in the gut. Taking pro-biotics at this stage is no good because if there is so much bad bacteria on your gut wall - there is no space for the good bacteria to live.
Increasing your fibre intake (like I did) will just make the bad bacteria thrive.
First you need to kill the bad bacteria and to do this I was put on a fibre-free diet for 2 weeks. Basically all the stuff you're usually told is unhealthy - white bread no crusts
potatos - no skins. The cramps and other symtoms almost instantly subsided - it was miraculous!!
After this I had lots and lots of probiotics - yoghurts and drinks etc to increase the good bacteria in my gut and slowly reintroduced fibre.
It really worked - I couldn't believe it after trying so many other things!
Anyway, whenever my IBS symptoms get really bad (luckily not very often now) I just follow the advice she gave me. I also avoid onions, leeks, chives and shallots like the plague.
I have set up a group on facebook (just search for "onion allergy/intolerance") or click on my website. I have started compiling some recipes and a list of onion-free supermarket finds on there. I hope people will find it useful and maybe help me add to it!