Onions:[1] [Rambam/Achronim]

Onions are unhealthy, and should never be eaten in the summer, and even in the summer should only be eaten on rare occasion.[2]  Some say in the name of the Baal Shem Tov that one should never eat raw onions, even in a salad, as eating them causes many illnesses.[3] The Talmud[4] states a person is not to eat onions due to the “snake” that is found within it. Some[5] explain this to refer to the sharpness of the onion which is considered similar to a poison which burns [and hence according to this approach one should never eat onions]. Others[6] explain this to refer to the middle part of the onion where the seeds are found in similar to a stalk. This part of the onion is referred to as the snake of the onion and should not be eaten. Others[7] explain that this refers to the stalk of the onion. Others[8] explain that this refers to the leaves of an unripe onion which has yet to grow one Zeres [i.e. three Tefachim]. Nonetheless, if one ate it and then drank beer, then the danger dissipates.[9]

Memory loss:[10] Some sources record that onions cause memory loss of Torah learning.[11] However, some Poskim[12] rule that there is no need to worry about this.

[1] See Peri Chadash 116:9; Kaf Hachaim 116:88; Sefer Shemiras Haguf Vihanefesh [Lerner] 4

[2] Rambam Deios 4:9; Kitzur SHU”A 32:14; See Sifri Behalosecha that onions are bad; Nedarim 66a that onions are bad for the heart; See all sources in coming footnotes!

[3] Sichos Haran 265; Baal Shem Tov Al Hatorah Yisro 46

[4] Eiruvin 29b, brought in Peri Chadash ibid; Aruch Hashulchan 116:14; Kaf Hachaim ibid

[5] Rashi Eiruvin ibid

[6] Tosafus Eiruvin ibid in name of Rabbeinu Chananel; Aruch in name of Rav Sherira Gaon

[7] Tosafus Yom Tov 7:4

[8] Admur 386:4; Eiruvin ibid; Machatzis Hashekel 386:13; M”B 386:22;

[9] Eiruvin ibid; Aruch Hashulchan 116:14

[10] Sefer Shemiras Haguf Vihanefesh [Lerner] 11

[11] Pesikta Zutrasa Parshas Behalosecha on Pasuk 11:5 “We remember the squash and…”, brought in Lekach Tov and Yalkut Reuveini; See there Pirush Rashi and Even Ezra

[12] Alei Terufa [Malbim] in Eretz Chemda Deios 11; See Sefer Shemiras Haguf Vihanefesh [Lerner] 11 footnote 2