Cut the narthangai into half and squeeze the juice and keep it aside.
Grind the ingredients given for rasam powder coarsely.
Heat a kadai under medium flame.
Add oil and temper with mustard seeds, curry leaves and asafoetida.
Reduce the flame and add the ground powder and fry for a minute.
Mash the chopped tomatoes in hand and add them to the kadai.
Add green chilli, turmeric powder, jaggery and required salt.
Add required water and cook the tomatoes well.
When the foam starts forming on top, switch off the flame.
Garnish with coriander leaves and pour the juice.
Mix well and serve with hot rice.