What the Scriptures and Sages Say About Onion and Garlic?

1. Lasunam gṛnjanam tvaivapalāṇdum kavakāni caabhakśyāṇi dvijātināmmedhya prabhavāni ca- Translation: Garlic, leeks and onions, mushrooms and all those plants that spring from impure substances, are unfit to be eaten by twice-born men. 

(Manu-Samhita Chapter 5, Verse 5) 

2. palandu lasunam sigrum alambum grjanam palam
bhunkte yo vai naro brahman vratam candrayanam caret 

Translation: O sages, one who eats garlic, onions, sigrum (a kind of plant), turnips, bottle gourd and meat, that person should observe a candrayana fast

(Padma Purana, Brahma Khanda 19.10) 

3. palandulasunadini jagddhva candrayanam caret
sraddhe devanpitrnprarcya khadanmamsah na dosabhak 

Translation: By eating garlic and onion one becomes sinful and as atonement one should perform Candrayana. (Garuda Purana 1.96.72) 

4. Onion, feces-thriving pigs, Selu, garlic, Goplyusa (milk of a cow before the lapse of ten days from calving), Tanduliya (a grain growing in faecal rubbish) and mushrooms— all these are to be avoided. (Skanda Purana 40.9) 

5. (But) he should avoid stalks of lotuses, safflower, gold or silver, onion, garlic, sour gruel, a thick fluid substance; so also chatraka (a kind of mushroom), vidvaraha, greasy milk of a cow during the first seven days of calving, vilaya (a particular product of milk) and mushrooms. By eating the small red variety of garlic, blossoms of kimsuka, a gourd, so also udumbara, bottle-gourd, a twice-born becomes fallen. (Padma Purana Vol. 42, 4.56.19b -24) 

6. I shall now mention those things that should always be avoided in Sraddha. Garlic, onions, globular radish and Karambha (groats or flour mixed with curds), and other articles devoid of smell and with no taste should be avoided in Sraddha.The reason is also mentioned (as follows): Formerly in the fight between devas and asuras, Bali was defeated by suras. From his wounds drops of blood gushed out and from them these articles grew up - hence they are avoided in the Sraddha rite. (Varaha Purana 16.11b-15) 

7. The expiation for having eaten the prohibited food is fasting. One should do the stiuka expiation for having eaten bhustma (a kind of fragrant grass) and garlic. (Agni Purana 173.37)

8. "Onion and garlic are EXTREMELY tamasic. They are as BAD as MEAT. Therefore, they are TOTALLY forbidden to  THOSE who wish for MORAL and spiritual GROWTH," Swami Sivananda (15th World Vegetarian Congress, India, 1957)

9. Swami Sivananda: "...Onions and garlic are worse than meat..." (pg. 119, Practice of Brahmacharya)

See also: http://sundaysatsang.blogspot.sg/2012/07/what-islam-says-about-eating-onion-and.html