Place lentils and turmeric in a medium saucepan with the measured boiling water and bring to the boil. Reduce the heat to low and simmer gently for 40 minutes, skimming off any scum that rises to the surface and stirring often. Use a balloon whisk to whisk the mixture until fairly smooth.
Heat the oil in a wok or frying pan over a medium heat. Add the mustard seeds and as soon as they begin to 'pop' (after a few seconds), add the curry powder, chillies and bay leaf. Stir-fry for 5–6 seconds until the chillies darken in colour.
Add the cooked lentils and measured water, and season to taste. Stir to mix through. Add the tamarind paste and agave syrup, and bring to the boil. Reduce the heat to low and simmer gently for 8–10 minutes. Stir in the coriander and serve immediately.