Why can we not store a cut-up onion in the refrigerator? And does adding a pinch of salt make it OK?

Leaving a cut up onion in the fridge does not appear in the Shulchan Aruch (Code of Jewish Law). Though it does have foundations in the Gemara and in the Zohar for kabbalistic reasons. The problem exists when the onion is peeled and without any spices. Therefore if the onion is un-peeled or it has an amount of spices (which may change the taste) then it is permitted. Therefore the ammount of salt depends on the size of the onion, though it is likely a bit more than a pinch might be needed. Rabbi Binyamin Bemberger