Onions

Yes, the humble onion.
Under-estimated, it is.
Why?

Here's a small tip about why leftover onions are a reason for FOOD POISONING.

Ed (a nutritionist) says that when food poisoning is reported, the first thing the
officials look for is when the 'victim' last ate ONIONS and where those onions came from (in the potato salad?). Ed says it's not the mayonnaise (as long as it's not homemade Mayo) that spoils in the outdoors.
It's probably the onions, and if not the onions, it's the POTATOES. He explained, onions are a huge magnet for bacteria, especially uncooked onions.
You should never plan to keep a portion of a sliced onion. He says it's not even safe if you put it in a zip-lock bag and put it in your refrigerator.
It's already contaminated enough just by being cut open and out for a bit, that it can be a danger to you.
Ed says if you take the leftover onion and cook it like crazy you'll
probably be okay, but if you slice that leftover onion and put
on your sandwich, you're asking for trouble.
Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down.

And here's a little reminder for those of you who have dogs.

Dogs should never eat onions.
Their stomachs cannot metabolize onions.