I don't attend feasts due to my allergies because that would be cruel and unusual punishment for any cook. ;-) However, there are people with fewer food allergies that attend feasts. Given that, I know many cooks don't use period ingredients but I'm wondering how you choose what ingredients to use and how closely you read those ingredients? For instance, I can't have margarine because nearly all of them have buttermilk or milk in them and those that don't - vegan- have corn. Now, granted, someone like me with both allergies is a long shot but I do know of others that have a milk allergy in the SCA. I typically just use olive oil as a replacement for butter in any period recipes -as they did in period- but I know olive oil might be too expensive for a large feast.
So, as a cook, how do you deal with the food allergy - ie, something that is not a choice (trust me, I'd happily eat cheesecake all day if the ending result was not the ER)- when creating a feast?