[QdoUOTE=quiltingcandy;7671525]I doubt that an onion would cause a problem - there are eggs and oil in mayonnaise which is much more likely to go bad if it gets warm. There are preservatives in mayo.[/QUOTE]
Does this mean when I use only half an onion I don!t need to toss the other half. That's what I always do as I only buy those white onions and around here those are at least baseball sized.