If a cut onion was left overnight soaking in water with no spices is it ok to use?
If a cut onion was left overnight it should not be used.
The Gemara (Niddah 17) writes that eggs, onions, and garlic, which are peeled and left overnight (without before being mixed with other ingredients) may not be eaten, because an “evil spirit” rests on them.
This halachah is not mentioned by the Rambam or the Shulchan Aruch, perhaps because they reasoned that the ways of ruach ra’ah (the evil spirt) have changed, yet it is mentioned by the Semak (168), by the Peri Chadah (116), and by the Shulchan Aruch Harav (Shemiras Haguf Vehanefesh), and cited by later authorities.
Therefore, if a cut onion is left overnight, it should be discarded. Placing the onion in water does not help – but if some of the peel remains, or if the onion has been made into “onion salad” by mixing with oil and spices, the onion can be left overnight.