Peel and finely dice the onions, ginger and garlic. Heat the oil in a large pot. Brown onions, ginger, garlic and lentils in it, season with curry, cumin and cardamom.
Add stock, bring to the boil. Simmer at low heat, stirring frequently, for about 15 minutes. Season to taste with salt, pepper and sugar. Wash parsley, shake dry and chop. Arrange the soup in bowls, add a blob of yoghurt to each bowl.
Sprinkle with parsley and curry.