If you go on a visit and you are served a meal along with other visitors, say, “thank you” before you start asking questions about the ingredient used in preparing the meal to confirm mushroom is not one them.
If you are told that mushroom is part of the ingredient used, do not immediately show you are allergic to it and irritate others. Courtesy demands that you smile and wait a few seconds, then excuse yourself and ask to use the restroom/bathroom.
On your way to the bathroom, you can alert the person that’s walking you about the issue at hand, before you are through using the bathroom another meal will be served you without anyone noticing. You do not want to grab unnecessary attention at table. Correct?