The dark green part of the leaves is the tough part that must be discarded. The closer you are to the center of the leek, the more light tender part of the leaf there is. If you simply remove the top of the leek where the leaves turn dark green, you’ll also lose some of the tender leaves inside. That’s why I remove the dark part one layer at a time. Use a sharp chef’s knife to cut one layer of leaves at a time where the whiter tender part ends and the tough dark green part begins.