The answer is that you must bring your own food to gatherings involving food, because you can only safely eat something that you have prepared. Communicate with the host beforehand: “I am on an extremely restricted medical diet because of my allergies, so I need to bring my own food. Will that bother you? I really don’t want to impose or make a big deal about it, but until I get my diagnosis sorted out, it is vital that I only eat food I’ve prepared myself.”