Melt the butter in a wide, heavy pan and put in the sorrel leaves, stripped of any coarse stalk. Allow the leaves to wilt briefly. Put in the potatoes. Add the garlic and chervil. Pour over just enough stock to cover the potatoes.
Bring to the boil, cover the pan and then simmer for 10 minutes. Remove the cover and allow to cook until the potatoes are soft and the liquid is reduced to a thick sauce.