For the potato-leek soup, clean and wash the leeks and cut them into fine rings. Heat 2 tablespoons of butter in a saucepan, add the leeks and sauté until translucent.
Wash and peel the potatoes, cut them into cubes and add them to the leek. Pour in the soup and cook at high temperature for about 15 minutes.
Fold in the creme fraiche and the remaining butter and refine with the chervil leaves. Season to taste with salt and pepper.