A classic side dish to any curry.
Cover the lentils with the water and leave to soak. Coarsely chop half the onion and finely chop the rest. Cut half the tomatoes into small chunks and finely dice the rest. Heat half the ghee in a heavy-based pan. Fry the garlic, finely chopped onions and tomato for 5 minutes, until the mixture has cooked to a golden-brown paste.
Pour in the lentils and their soaking water and bring to the boil. Add the turmeric, chilli powder and coconut milk and simmer for about 30 minutes, until the red lentils have broken down (the green ones will stay intact). Season to taste with some salt and remove from the heat.
When you're ready to eat: Heat the rest of the ghee in a large, deep frying pan. Add the mustard seeds, cover the pan with a lid and fry until they pop. Add the ginger, the rest of the onions and tomatoes and the chillies. Cook for 5 minutes and then pour everything into the lentil mixture. Add the coriander and stir well.