Hi Rabbi,
I bought a red onion that is very big, and I cut only a piece of it. So here are my questions;
1. I read that it might be OK to leave the rest overnight if the outer skin is left on a small part of the onion, or it is covered with only a few drops of oil or even salted only a bit, is that kosher? How much skin, oil or salt are needed? I also heard that nowadays since the nature of things has changed it is permitted at least bediavad when already done, and one does not have to throw them out, is that correct? How about is it was left just in a sealed container or in the fridge?

A. 1) The source of this Halacha is in Talmud (Nida 17a) that rules one may not eat a peeled egg, onion or garlic clove that was left overnight, because of the ruach ra’ah (spirit of impurity) which rests upon them. This applies even if it was left in a refrigerator or other sealed container. (Hakashrus 18: 11, however Rivavos Efraim 3: 495 permits a left over onion wrapped in aluminum foil)
Shevet Halevy (6: 110: 5) and others maintain that the reason of not eating the above items is not because they present today a physical danger, but rather a spiritual one based on the ruach ra’ah they posses. Tosafos (Yuma 77b) and others opine that this type of prohibitions have changed with the times. The fact that the Rambam, Tur and Shulchan Aruch omitted these prohibitions, is indeed a reflection of the change (See also: Yam Shel Shlomo Chullin 8:12, Mordechai Shabbos p. 161, Magen Avraham O.H. 173:1, Darchei Teshuva 74, Kaf Hachaim Y.D. 116:92, , Minchas Yitzchok 2:68:13, 6:74, Oz Nidberu 11:47, et. al.)
Horav Shlomo Miller’s Shlit”a opinion is that many Poskim maintain that this halacha should be observed and that is still the minhag of many today. (See also; Shulchan Aruch Harav Shemiras Haguf 7, Daas Torah 513: p. 639, Sdei Chemed Lamed klal 141:31 p. 372, Ta’amei Haminhagim (lekutim) page 497:16, Lekutei Halachos Chofetz Chaim Nida ibid. Ben Ish Chai Pinchas 2:14, Maharsham 4: 148, Darchei Teshuva 116:74, Kaf Hachaim 540:1, Igros Moshe Y.D. 3:20, Be’er Moshe 3:115, Yabia Omer Y.D. 2:7, Ha’kashrus K’halacha page 243:footnote 50, Orchos Rabbeinu 1: p. 210: 16).
However, one may be lenient on the amounts of skin, oil or salt needed and if they are noticeable the food is permitted. When in need, after the fact one can also be lenient.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a