Omg! I should have constant food poisoning or worse! I rarely use a whole onion at one time...what I don't use is put in a ziplock and set in fridge....I'm in good health.....[/QUOTE]

I have always stored my onions in zip lock bags and have never ever gotten sick. Where is this information coming from ?

I have always stored my onions in zip lock bags and have never ever gotten sick. Where is this information coming from ?[/QUOTE]

I can't answer your question, but we always keep our onions at room temperature until we cut into them. Then we chop the leftover onion, put it in a refrigerator dish and use as needed. We've never been sick from this (46 years of doing this.)
The mayo IS the problem with the potato salad. Leave some mayo in a dish outside in hot weather and check it in a couple of hours. Would you want to eat it? Always refrigerate or keep cold anything made with mayo or uncooked eggs(it's actually better not to even eat anything with uncooked eggs).
Wherever this info about the onions came from might be for onions that have dirt on them. That's all I can figure.

All urban legends. Being raised Italian, with all of the onions that we used, I would be dead by now and not turning 66. With properly covering the onion and refrigeration, you can eat onions left over in the refrigerator for at least a week, if not more.

If you are really worried about it, chop up the rest of the onion, put it in a Ziploc and freeze it. You can then put the leftovers in casseroles, stews, soups or anything that will cook the onion really well.

Will try this as sometimes I like to put onion in my egg salad. It usually takes planning. I try to remember to set a little onion aside (to use in my egg salad) when I'm making onion & mushrooms. Of course I rarely remember to do this.

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.

Also, potatoes are a starch, and starch is sugar, and sugar attracts bacteria.

I don't throw away any part of an onion-- I do keep them refrigerated after cutting and have never had a problem. I think a lot of these scare messages are not to be believed. 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.[/QUOTE]

I usually chop up the unused onion, spread it on a cookie sheet and freeze it for about 20 minutes. Then I put it in a freezer bag and return it to the freezer. I can take out what I need for omlets, etc. Better than buying the already frozen, chopped onions!
But if I know I'm going to use the remainder of an onion soon, I just store it in the fridge.

As I remember from childhood, there was belief that putting a cut onion in the room would help purify the air. Whether it did or not, I don't know but, like others here, I store a cut onion in the refrigerator and haven't gotten sick. When my son was school age, he wanted to take lunches with him but it got difficult coming up with a variety of sandwiches, etc. A home economist recommended using Miracle Whip instead of mayo saying it is salad dressing and not the same ingredients as mayo. She said it was safe to have unrefrigerated during those hours between prep and consumption. Probably best to include a chilled container in the lunch bag though?? I wouldn't trust this theory for salads for picnics as the entire salad would be getting warm, including the eggs. But to spread on a sandwich, apparently Miracle Whip is safe. (He's still alive anyhow!)

There is a German potato salad , that is served warm. Are there different ingredients in that?