How sensitive are you? I’ve found I struggle with them a bit if I eat lots suddenly after not eating them for a while, but if I continue, it tends to be fine. My guess is that the reason it is inconsistent for me is related to fluctuations in gut bacteria partially due to what I am eating…
I know actual IBS is no joke, but I think it would be interesting to determine your threshold and see if you can change it at all. For example, eat a pea-sized piece of onion and wait a couple days. No effect, start eating a pea-sized piece of onion every day and wait for cumulative effects (a week or two?). Still no effect, add another piece for one day, go back to regular dose for a couple days, etc. When you reach the point where it starts to cause discomfort, back off a bit and try to push it again in a week or two.