Clean, wash and cut the leek into rings. Peel and chop the onions. Peel and wash potatoes and cut into small cubes. Put chicken stock, potatoes, onions and leek in a pot, bring to the boil, cover and cook for about 20 minutes.
Add butter and mash coarsely with a potato masher. Press small dumplings from the sausage directly into the soup. Simmer again for another 8-10 minutes at low heat. Season with salt and pepper.
Meanwhile wash parsley, shake dry and chop finely. Stir 3/4 of the parsley into the soup. Serve the soup in small bowls, sprinkled with a dash of crème fraîche, nachos and 1/4 of the parsley.