In Jainism, avoiding onions (and other root vegetables) is tied to core principles of ahimsa (non-violence) and careful living.

• Root vegetables harm many life forms: When you pull up a root plant, you often kill many tiny organisms in the soil and may destroy life that would have grown there. Jains prefer not to cause unnecessary harm to living beings, even microscopic ones, by uprooting roots.
• Violence and karmic impact: Uprooting and consuming root vegetables is seen as causing more violence to life. Jains aim to minimize violence in daily choices, so they avoid foods that involve harming many life forms.
• Purity of practice: Root vegetables can carry more pests and require scrubbing/processing, which can cloud mindfulness and discipline in daily worship and ethical conduct.
• Tamasic/rajasic effects: Onion is traditionally considered to have a strong stimulating (rajasic) and impure (ta’mag) influence in some Jain understandings, which some seek to avoid to maintain mental clarity, gentleness, and focus on spiritual practice.

Differences between Digambar and Shwetambar:

• Both traditions generally discourage root vegetables, including onions, as part of a strict observance of ahimsa.
• There can be regional or community-level variations in strictness or implementation, but the underlying reason—non-violence and simplicity—remains the same.

If you’d like, I can share scriptural references and the precise wording used in Jain texts to explain this practice in more detail.