Peel the ginger and dice it. Chop the garlic, cut the lemongrass into small slices. Crush the cumin, coriander and chilli in a mortar.
Chop the onion and sauté in the oil. Add ginger, garlic and lemongrass and sauté. Add the mustard seeds until they start to burst, briefly fry the remaining spices. Stir in the lentils and roast them until they are coated with the oil.
Pour 1-1.2 liters of water, bring to the boil and simmer on a low flame until the lentils start to disintegrate. Season with salt, remove the cinnamon stick and mix up with the hand blender if necessary.