Onions peel and chop finely. Peel, wash and cut the potatoes into small cubes. Heat the oil. Brown onions and potatoes in it. Deglaze with 650 ml water. Stir in stock. Bring everything to the boil and simmer covered for 15 minutes.
Wash the herbs and pluck the leaves from the thick stems. Add to the soup and simmer for another 5 minutes. Remove from the heat and stir in the cream. Puree the soup finely with a hand blender. Season to taste with salt, pepper and lemon juice.
For the yoghurt, finely crush caraway and fennel in a mortar and stir into the yoghurt. Season to taste with salt and pepper. Serve the soup with yoghurt.