Heat the oil in a large saucepan over medium heat. Add the mustard seeds. As they start to sputter add the curry leaves and tomato then season with cumin, pepper, chilli flakes, turmeric and garlic. Pour in the water and bring to a boil. Stir in tamarind paste, adjusting to taste if you want, and season with salt. Simmer for about 2 minutes.
Ladle into bowls and garnish with coriander to serve. Drink it like soup or eat it with rice.