Put the dal into a large saucepan and add water to cover by about 4cm. Add all the remaining dal ingredients, bring to the boil, then lower the heat to medium and simmer for 45-60 minutes. The dal should be soft but still with a little bite. Use a potato masher to break up about half of the lentils, being sure to leave plenty of texture.
For the tarka, heat the oil in a pan over a medium heat, add the mustard seeds and fry for 30 seconds until they pop. Stir in the shallots, Kashmiri chillies and curry leaves and fry for 2-3 minutes until the shallots are softened and golden.
Spoon the tarka on top of the dal, sprinkle with fresh coriander leaves and serve.