Add tamarind water (soak tamarind in a luke warm water for about 20 -30 minutes or microwave tamarind with little water for about 30sec to 1 minute), chopped tomato, chopped green chillies, cilantro, curry leaves and add enough water to get the required consistency.
Add a pinch of Haldi and Hing and get the mixture to a boil.
When it starts boiling add freshly prepared rasam powder (about 2 Tbsp), jaggery and salt to taste, and let it boil for a 3 to 5 minutes.
Put oil in a fry pan, when it gets warm, add mustard seeds. After it sputters add dry red chillies (split into pieces) and hing (if desired) and curry leaves.