Rinse the dal well in cold water, then put it in a small pan with 250ml cold water and the green chilli, slit down its length (if using two chillies for a spicier result, finely chop the second one and add that to the pan later at the same time as the garlic and ginger).

Bring to a boil, turn down the heat and simmer gently, covered, for about 30 minutes or until very soft. Lift out the whole chilli. Do not drain the lentils.

Heat a half-tablespoon (a generous teaspoon) of your chosen fat in a saucepan, add the ginger, garlic and tomatoes, and cook until the tomatoes dry out and audibly begin to fry.

Stir in the turmeric, pepper and a good pinch of salt, cook for just 30 seconds, then add the dal and its cooking water, as well as another 400ml water, or enough to make a thin broth. Bring to a simmer and add the tamarind paste – brands vary greatly in strength, so you may need to add more later. Simmer for five minutes, then take off the hob. This can be made in advance up to the tempering, which should be done just before serving.

Put the remaining fat in a small frying pan on a medium-high heat. Once it’s hot, add all the remaining dry spices and curry leaves and cook, stirring, until the mustard seeds begin to pop.

Stir this into the hot soup, and taste and adjust the seasoning as necessary. Divide the hot rasam between bowls or cups, top with the chopped coriander, and serve with hot steamed rice on the side, if you like.