Heat the oil in a large pan, sauté the onion for approx. 3 mins. Add the garlic and ginger, cook for a further 1 min. Add the curry, turmeric and cumin, continue cooking for 1-2 mins. Add the lentils, coconut milk and water, bring to the boil, season with salt, simmer for approx. 15 mins. until the lentils are soft. Stir occasionally.
Heat the oil in a frying pan, add the mustard seeds, cover and toast for 10-15 mins. Add the mustard seeds to the dahl, stir thoroughly. Serve the dahl immediately.