Soak the tamarind in just enough hot water to cover for 10 minutes, crushing with the back of a spoon to extract the pulp. Strain through a sieve into a bowl and set aside. If using tamarind paste, dissolve in 100ml warm water.
Heat the ghee or oil in a pan over a medium heat. Add the mustard seeds and curry leaves and cook until the mustard seeds begin to pop.
Add the crushed cumin, dried red chilli, crushed black pepper and asafoetida. Stir and cook for 30 seconds until fragrant.
Add the chopped tomatoes, green chillies, ginger, lentils, turmeric and salt. Stir to combine.
Pour in the 750ml of water and the strained tamarind water. Stir well.
Bring to the boil then reduce to a gentle simmer. Cook for 20 minutes until the lentils are completely soft and have dissolved into the broth.
Taste and adjust seasoning — add more salt, tamarind or chilli to taste.
Serve garnished with fresh coriander, either as a soup or poured over rice.
