I didn't know about mushrooms either but I never eat anything with onions in. They smell lovely but meowmy says they can cause/affect anaemia (can't remember which).
We had a lovely kitty called "hope" who we lost last year (before they brought me home so I never met her). She is meowmy's inspiration for doing work with rescues. She had renal failure and never responded to treatment. It was a bad few months and meowmy spent every available minute on the internet trying to find advice and support (sadly she didn't know about purrs back then).
She found lots of eye opening things about plants and seemingly harmless foods that can be deadly.
As a result, the only treats Josh and I have are meat and fish to be on the safe side. I'm allowed a little cheese on occasion and p.i.l.l.s are given to us using only beef paste or "Mum's Own" babyfood.
I think she's paranoid and I'm sure that most things are harmless in small doses, but it's better to be safe.
Shame really, meowmy loves quorn!