Heat oil in a pot and add mustard seeds, dried red chilies, curry leaves, and asafoetida. Let mustard seeds splutter.
Add chopped tomato and sauté for 2–3 minutes until soft.
Mix in tamarind paste, rasam powder, and salt. Stir well.
Add cooked dal and 2 cups of water. Mix everything and bring to a boil.
Simmer the rasam for 10 minutes so flavors blend well.
Adjust salt and spice as needed.
Garnish with fresh coriander leaves and serve hot with rice or as a soup.