Clean the leek, cut into rings (including the green one, if it is good) and wash. Drain. Peel the potatoes and cut into cubes. Pre-cook the sausages.
Put the leek, potatoes and sausages cut into rings into the hot oil and stir. Turn the heat down and simmer until everything is soft (about 20 minutes, do not add any more liquid). Salt, pepper and finally stir in the creme fraiche. Bring it to the boil again.