Rinse the red lentils in cold water until the water runs clear.
Cover with fresh water and set aside.
Using a mortar and pestle, roughly grind the cumin seeds and set them aside.
Then, grind the ginger, garlic, and green chile into a fine paste. Alternatively, use a small blender.
Heat the oil in a lidded saucepan over medium heat.
Once hot, add the curry leaves, mustard seeds, ground cumin, and black pepper.
When the mustard seeds pop and the curry leaves crackle, carefully remove a few leaves for garnish.
Add the ginger-garlic-chile paste to the pan and sauté for 3 minutes until fragrant.
Stir in the chopped tomatoes and cook for about 4-5 minutes, until they soften and become jammy.
Drain the soaked lentils and add them to the pan along with the tamarind paste and 3¼ cups (770ml / 26 fl oz) of hot water.
Bring to a boil, then reduce the heat, cover, and simmer for 20-25 minutes until the lentils are completely soft and cooked through.
Stir in the salt, tasting and adjusting as needed.
The final rasam should be a thin soup, both hot and sour. Ladle into bowls, garnish with the reserved crispy curry leaves, and serve immediately.
Enjoy it on its own or with a side of steamed basmati rice.
