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.
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.
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.