Peel and chop the onion. Clean and wash the leek. Cut the white of the leek into thin rings. Wash, quarter and core the apples. Cut 2 quarters into small cubes, sprinkle with lemon juice and set aside.
Cut the rest into large pieces. Heat 3 tablespoons of oil in a saucepan. Fry onion, leek rings and apples in it. Season with salt, pepper and sugar. Sprinkle with curry and sweat for about 1 minute. Deglaze with apple juice and stock, bring to the boil and simmer for about 20 minutes.
In the meantime cut the leek green into fine strips. Heat 1 tablespoon of oil in a frying pan. Steam leek strips and apple cubes for 1-2 minutes until translucent. Puree the soup and season to taste with salt, pepper, sugar, nutmeg and cream.
Pass the soup through a sieve into another pot and bring to the boil again. Arrange the soup in bowls, add 1 tablespoon of yoghurt to each bowl. Sprinkle with apple cubes and leek strips, dust with curry.