Cut the sausages into slices. Clean and wash the leek and cut into rings.
Melt the butter in a soup pot, add the leek and sweat for about 2 minutes, lightly salt. Add the meat stock, beans with their juice and sliced sausage. Bring the soup to a boil and cook for about 10 minutes. Mix the flour with milk, add to the soup, bring to the boil and simmer for a few more minutes.
Before serving, add finely chopped parsley and season the soup with salt, pepper and nutmeg to taste.