Step 1

Heat the oil and butter in a large saucepan. Add the leeks and cook over medium heat for about 3 minutes, stirring occasionally.

Step 2

Meanwhile, cut the potatoes into sticks about 3/4-inch thick and slice the sticks crosswise into 1/8-inch slivers (you will have about 3 cups). The potatoes should be kept in water after peeling, but they should not be washed after they are cut into slivers; this would wash away their starch, which helps make the soup smooth.

Step 3

Add the stock, water, salt, and pepper to the leeks, then mix in the potato slivers. Bring to a boil, reduce the heat, and boil gently for about 12 minutes.

Step 4

Serve the soup with a sprinkling of the parsley or chervil on top and, if you like, the grated cheese or a dollop of butter.