Heat a pan with 1 tsp oil first, add cumin seeds and green chillies. Sauté them for a few seconds until you see small bubbles on green chillies. keep aside.
Clean and chop the celery into ½ inch pieces.
In the same pan, heat 2-3 tsp oil, add chopped celery, fry them for a few minutes.
Add chopped tomatoes, cook them until the tomatoes are mashed, water is completely absorbed and the celery becomes soft. Let it cool.
Grind green chillies, jeera and salt. Then add the celery, tomato mix. Blend it to a smooth paste.
For the tadka/tempering
Heat oil in a pan. Once hot, Add the mustard seeds, jeera, 2 broken red chillies, hing and few curry leaves. Let the mustard seeds splutter and pour this mixtre on the chutney and mix.
Serve with Idli, dosa, rice or roti.
Notes
You may add slightly roasted chana dal, urad dal and coriander seeds while grinding.
You can replace green chillies with red chillies if using dals and coriander seeds.
Add tamarind juice or lemon juice or raw mango for more sourness.