Heat 2 tablespoons of butter in a large saucepan, add the leek rings, salt and sauté until translucent.
Wash, peel and slice the potatoes and add to the leeks. Fill the whole thing up with 1 1/2 liters of water and cook for 15 minutes at a high temperature.
Wash the chervil and pluck the leaves off. Pull the creme fraiche and the remaining butter into the soup, add the chervil.
Season the soup with salt and pepper and serve immediately.