Q. Why should we avoid Onion to eat? I don’t eat onion. I want to know so that I can convince other persons.
A. Onion and garlic come under the category of foods in the mode of passion and ignorance. Therefore, they cannot be offered to Krishna. They also have a foul smell, a sign of tamasic food.
There is also a Vedic reference: The Manu-smriti (5.5) says, lasunam caiva palandu abhaksyani—“Garlic and onions are unworthy of eating.”