In a large saucepan, heat the oil and saute the leek. Add peas, potatoes, marjoram and parsley. Simmer, covered, 25 minutes, stirring so it doesn't stick. Add a little stock if necessary.
Puree with a hand blender, and add the rest of the stock. Season and bring to a simmer.
Add the creme fraiche and reduce heat so it does not boil. Add the frankfurters and simmer until cooked. Add a dash of vinegar and serve, sprinkled with chervil.