Soak the lentils in boiling water to cover for 10 minutes. Drain and put into a large saucepan with the measured hot water. Bring to the boil over a high heat, spooning off any scum that comes to the surface. Reduce the heat and cook for 20 minutes or until soft and tender.
Drain the lentils and process to a purée in a food processor or using a hand-held electric whisk. Return the purée to the rinsed pan with the tomatoes, chillies (if using), turmeric, ginger and fresh coriander. Season with salt and pepper, return to the heat and simmer gently for 5 minutes.
Meanwhile, make the tarka. Heat the oil in a small nonstick frying pan and add the remaining ingredients. Fry, stirring constantly, for 1–2 minutes.
Remove the tarka from the heat and pour on to the cooked dhal. Stir and serve hot with basmati rice, natural yogurt, if liked, Naan (see