Heat a pan/kadai adding cooking oil, mustard seeds, split black gram lentil, broken red chillies and curry leaves. When mustard seeds splutter add chopped tomatoes and stir for a while.
When tomatoes are mushy, add turmeric powder, jaggery, slit green chillies. Saute and add one to one and half cup of water to it. Let it boil.
Now add udupi rasam powder, dissolved asafoetida and salt. Mix well. When froth starts forming switch it off.
Instant tomato rasam is ready. Garnish with chopped coriander leaves and serve it with hot steamed rice topped with ghee.