Boil chopped tomatoes with tamarind paste, salt, and water until soft, then mash well.
Crush cumin seeds and garlic paste together into a coarse paste.
Add crushed garlic-cumin paste and black pepper powder to the tomato broth; simmer gently for 8 minutes.
Heat ghee in a small pan, crackle mustard seeds, curry leaves, and asafoetida.
Pour hot tempering into the rasam and serve piping hot.