Add the tomato paste to the water and whisk untill well combined.
In a pot do the tempering by heating the oil. When the oil gets hot, add the mustard seeds.
Once the mustard pops, add the cumin seeds, red chilly, curry leaves and hing. Fry for 20 seconds.
Add the tomato paste and water mixture to the tempering.
add in the rasam powder, salt, turmeric powder and sugar.
Boil the rasam in medium heat for 12- 15 minutes and check for seasoning and adjust.
Finally add a tea spoon of lemon juice and chopped coriander and turn off the stove.
Serve the rasam with hot white rice.