I sympathise, onion, garlic and leeks are my kryptonite and are included on FODMAP list of foods to avoid. I used to be able to eat them fine but for about the last 10 years, when I've eaten them I wake up in an agitated state and feel drained the next day. So I avoid them. There is a good range of sauces available from Bays Kitchen which are tasty without onion or garlic. Expensive but to add variety and a bit of spice to my diet they're worth it. www.bayskitchen.com
I'm not convinced that stress is the cause of IBS although of course it is not unusual for periods of stress to affect our digestion. I know that if I avoid certain foods I avoid flareups. Equally, if I inadvertently eat certain foods I try to avoid (or am tempted to eat certain foods I should through experience avoid) I feel very under the weather and find it difficult to deal with any stress. I think stress causing IBS gets blamed a lot as, other than avoid certain foods, there doesn't seem to be a failsafe treatment for it. It's also interesting that a lot of women develop it later in life, at a time when they're retired when life is generally much less stressful but perhaps the digestive process has deteriorated through general aging.