Roast the lentils in a pan without fat for 4-5 minutes while stirring continuously. Pour into a sieve, rinse. Peel and finely dice onion, garlic and ginger. Peel, wash and chop the sweet potatoes.
Heat the oil in a pot. Fry onion, garlic and ginger for about 3 minutes while turning. Steam sweet potatoes briefly. Add the lentils. Add 850 ml water, stir in broth and bring to the boil.
Simmer at medium heat for about 20 minutes.
Roast the cashew nuts in a pan without fat until golden brown, take them out, let them cool down a bit and chop them roughly. Wash the coriander, shake dry and pluck off the leaves. Season soup with 1⁄4 tsp. ground coriander, 1⁄2 tsp. curry, salt, cayenne pepper and lemon juice.
Serve the soup with 1 blob of yoghurt. Serve sprinkled with coriander.