Jains don’t eat onion but do eat groundnut (peanut) because, in Jain dharma, they are treated as different types of plant parts with different levels of himsa (violence).

1. Why onion is not eaten

Onion is a root / bulb (kand-mool) and is avoided mainly for these reasons:

1. Uprooting kills the whole plant

- To take an onion, you usually pull out the entire plant from the soil. - This kills the plant and also disturbs/kills many tiny living beings (micro-organisms, nigod) in the soil.

1. Anantkay – many lives together

- Root vegetables like onion, garlic, potato, etc., are traditionally understood to contain many living beings in one body. - Eating them is considered more himsa than eating fruits, leaves, or seeds.

1. Effect on mind

- Onions (and garlic) are also considered to increase passions and restlessness, which is not good for a calm spiritual life.

Because of these reasons, Jains of both Shwetambar and Digambar traditions generally avoid onion.

You can read more here

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2. Why groundnut (peanut) is usually allowed

Groundnut grows under the ground, but what we eat is:

• Not the root, it is the seed/fruit (pod) of the plant.

From a Jain point of view:

1. Seed, not root

- Jain shastra allows eating fruits, seeds, grains, pulses, etc., which can be taken without necessarily killing the whole plant. - Groundnut is treated like other pulses/beans, just that it grows in the soil.

1. Less himsa compared to roots

- When you take groundnuts, you are mainly taking pods containing seeds. - The idea is that this causes less harm than pulling and eating the root itself (like onion, potato, carrot, etc.).

1. Accepted in both traditions

- In both Shwetambar and Digambar traditions, peanuts are generally permitted as normal Jain food. - Some very strict people or during special vrats may personally avoid them, but that is a personal extra vow, not a general rule.

You can read more here and here

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Simple summary

• Onion = root (kand-mool) → uprooting kills plant + many tiny lives → more himsa → so Jains avoid it.
• Groundnut = seed/fruit growing in soil → treated like other pulses/seeds → less himsa → generally allowed.

So the difference is not “underground vs above ground”, but root vs seed/fruit and the amount of violence involved according to Jain ahimsa.