Wash the watercress and parsley, shake dry and pluck the leaves from the thick stems. Peel onions and potatoes. Chop the onions finely. Wash and chop the potatoes. Heat 2 tablespoons of oil in a saucepan.
Sweat onions and potato cubes in it, then deglaze with broth and cream. Let the soup simmer for 15-20 minutes. Add watercress, except for a few leaves for garnishing, and parsley to the soup.
Let the soup simmer again for about 5 minutes. Puree the soup and season to taste with salt, pepper and possibly some lemon juice.
Finely crush the caraway and fennel seeds with a mortar. Mix the yoghurt with the spices, except for something to sprinkle, and season with salt and pepper. Arrange the soup, garnish with some spiced yoghurt and cress.
Add the rest of the yoghurt. Drizzle 1 tablespoon of oil over soup and yoghurt. Sprinkle with spices.