Prepare all the ingredients.
thinly the leeks that have been rinsed under running water.
In a stockpan, melt the butter.
Add the leeks...
...and for a few minutes without browning.
Meanwhile, cut the potatoes into pieces or slice them.
Add them to the sweated leeks...
...and cold water.
Bring to a boil and add salt and pepper. Cook for about 30 minutes.
When finished, mix the preparation to obtain a fine smooth soup.
Add crème fraîche...
...and mix using a whisk.
Add a large knob of butter...
...and mix thoroughly to incorporate it. Serve very hot.