Method

Coarsely crush black pepper and cumin seeds (do not powder — a rough crush is preferred for texture). Blend tomatoes. In a pot, combine tamarind water (about 400ml diluted), blended tomato, turmeric, salt, and crushed garlic. Bring to boil and simmer 8 minutes.
About 10 min
Tip: A slightly coarser grind of pepper and cumin is the hallmark of good rasam — fine powder tastes different and less aromatic.
Add the coarsely crushed pepper and cumin, and the optional mashed dal. Simmer 5 minutes. Add asafoetida. Taste — the rasam should be thin, very tangy, spicy from pepper, and aromatic from cumin. Add more tamarind water or salt as needed. It should be a flowing, thin broth.
About 5 min
Tip: Rasam should be thinner than sambar. If it's too thick, add boiled water. Good rasam should be drinkable.
Make tempering: Heat ghee in a small pan until smoking. Add mustard seeds (let pop), dried red chillies, curry leaves. Pour immediately over the rasam. Cover. Add fresh coriander. Serve hot in bowls as a soup course, or mixed into rice. Best drunk piping hot.
About 3 min
Tip: Drinking rasam straight from a cup is the authentic way — pour into a stainless-steel tumbler and drink it as a hot beverage.

Nutrition per serving (estimate)