Same here for me. I have never thrown away the unused part of an onion in my life and I am 68 years old. I put the unused portion in a baggie in the fridge, only throw it out if it happens to start looking dried out or "old". We use onion a lot in my house. I think the main thing with potato salad is to keep it chilled without worrying about exactly which ingredient may go bad. Just follow food safety guidelines for all of your food.