Soak tamarind for 10-15 mins. Squeeze the juice out and keep the plain tamarind water. Mash a medium sized tomato into the tamarind water. Add pepper and cumin powder (2-3 tbs each) and salt. keep it aside.
Heat the pan, add oil, mustard seeds, cumin seeds, crushed garlic flakes, curry leaves, red chillis, few pieces of crushed small onions. After sauting it for few mins add the tamarind juice in it and add asafoetida powder for th aroma and keep watching the rasam, so as to not let it boil. The moment you see it starts boiling switch off the burner. Simple rasam is now ready!