In Jainism, masoor dal (red lentil) is generally:

• Allowed for most Jain householders (shravaks), as long as:

- It is dry/old grain, not fresh green. - It is not sprouted. - It is cooked fresh, in the day, following normal Jain rules (no onion–garlic, etc.). ( jainknowledge.com)

• It is even listed as permitted during stricter periods like:

- Paryushan (as “pulses/dals: … masoor (not sprouted)”) ( jainknowledge.com) - Chaturmas and Ayambil menus (boiled masoor dal) ( jainknowledge.com)

However:

• Some very strict Jains / some Digambar families avoid many legumes (including masoor) as a personal or community practice.
• Rules can vary by sampraday, guru, and local sangh.

So, in simple words:

Yes, a Jain can eat masoor dal in most traditions, provided it is dry, not sprouted, and cooked according to Jain rules. If your local guru or family tradition says to avoid it, then follow that guidance first.

You can read more detailed Jain dietary guidance here:

• General Jain food rules
• Question specifically about masoor dal and tomatoes