Short answer: In Jain Dharma, we do not eat onion.

Why:

• Ahimsa: Onion is a root (kand–mool). Uprooting it kills the whole plant and disturbs countless tiny lives in and around the root (anantkay/nigoda).
• Inner purity: Its strong, pungent nature is believed to excite passions and disturb equanimity needed for sadhana.

Traditions:

• Both Shwetambar and Digambar traditions teach refraining from onion (and other root vegetables). Monastics are strict; householders also avoid it, and during Paryushan/Chaturmas the rules are even stricter.

More to read:

• Why Jains avoid onion/garlic and other roots
• Why we do not eat root vegetables