Peeled Onions and Evil Spirits
In the course of a culinary discussion, my wife recently mentioned to someone that she was storing half an onion in the refrigerator. The other person was horrified, and informed my wife that this is categorically forbidden by halachah. Was she correct?
Rav Ari Kahn has a terrific shiur on this topic at YU-Torah, in which he says as follows: True, the Gemara does say that eating peeled onions (and garlic, and eggs) that were left overnight is lethally dangerous, due to the "evil spirit" that rests upon it. The Gemara further says that someone who does this is considered to be responsible for the ensuing loss of life. However, there are three factors which mean that this is not a halachah today.
First is that not many people since that time believe that there is actually any such danger. (Reinterpreting the Gemara to be referring to some sort of scientifically-confirmed phenomenon is problematic, since the danger is considered to be neutralized if even a tiny amount of peel is left on it.) And we are not just talking about Maimonidean rationalists; even Tosafos states that such "evil spirits" are no longer found.
Furthermore, whereas other such statements in the Gemara (based on views that are not consistent with contemporary science) may still be halachically binding due to their having been canonized in the halachic tradition, this is not the case with peeled onions. None of the major halachic works of the Rishonim or early Acharonim make any mention of this. Only recently did it become more common to find halachic works making mention of it.
Finally, it is certainly not part of the living tradition. How many of our mothers and grandmothers were ever concerned about such a thing, or even heard of it? Not many!
Thus, concludes Rabbi Kahn, if it's not mentioned in any of the major halachic works, and is not part of the general tradition, and is not a person's own family tradition, then while a person is entitled to adopt it as a stringency, you can't call it a halachah!
While I think that Rabbi Kahn's analysis is excellent, I'm not sure how long it's going to be accurate for. Due to the phenomenon of "chumrah creep", and the rise of the book tradition over the living tradition, the practice of not eating peeled onions that were left overnight is rapidly spreading. At some point, it is going to be considered normative practice in all circles. And at that point, it effectively becomes halachah. That's probably not a good thing, but it's near-inevitable. Such is the nature of Jewish evolution.
On an entirely different note: If you are coming to Israel for Chanukah, then (a) book a tour at the Biblical Museum of Natural History, and (b) let me know if you can bring some supplies for the museum from a US or UK reptile store!