I was at a nice enough restaurant last night, and they had two vegetarian mains on the menu. One had blue cheese involved, and I asked the waiter if it was possible to have it without the blue cheese, or if the mushrooms were pre-prepared that way. He assured me they weren't, and asked if I'd like a small amount of blue cheese so the taste wasn't too strong. I told him I'm dangerously allergic, and said again that if they were pre-prepared I'd take the other vegetarian entree and that would be fine.
Two of my three baked mushrooms were fine. The third had obviously had some blue cheese scraped off it. I couldn't see it or taste it, but the instant reaction (it looks like anaphylactic shock, only it isn't likely to be fatal unless I swallow my tongue) said it was there.