Take water and washed tomatoes in a pot and bring to boil.
Once the skin of the tomatoes begins to peel off, remove from water, remove the skin and puree tomatoes. Pour puree back into boiled water. Alternative is roughly cut peeled tomatoes, put them back in boiled water and blend using an immersion blender.
Grind all the ingredients listed under spice blend and add it to tomato puree mixture.
Add chili powder, curry leaves, ginger garlic paste, kothmir sprigs and onions (if using) to tomato puree mixture and boil for about 10 minutes or until raw smell of tomatoes and spices is gone.
Meantime, make the tempering by heating oil in a pan.
When oil is hot, add red chili, mustard seeds, cumin, curry leaves and ginger garlic paste.
Once seeds splutter, add turmeric and turn off the heat.
Pour it over tomato chaaru, cover and let boil for couple of minutes.
Taste rasam and adjust seasoning. For a tangy rasam, squeeze some lime juice. Squeeze lime juice just before turning off the flame or after turning off the flame.
Garnish with kothmir and keep the rasam covered until ready to serve.
Serve hot with steaming rice.