Sweat the onion in olive oil in a large saucepan. Add the dry lentils and toast briefly. Pour in the broth. Bring to the boil and cook for about 15 minutes. Season with cumin and chili powder to taste. Puree as desired.
The soup can be refined in three ways:
Option 1, vegetarian and rather Turkish:
Put about one tablespoon of yoghurt per person in the soup plate, fill up with soup and stir gently.
Option 2, vegan and more central European:
Fry the sour apple and one onion, put about one tablespoon each in a plate, fill up with soup.
Option 3, vegetarian and Mediterranean:
Dice the sun-dried tomatoes and feta, put the soup on plates, sprinkle with the tomatoes and feta.