 Cut through the leak lengthways from where the white starts to turn green, along to the leafy end. Turn over and cut again. Rinse the leak under cold running water, separating the inner layers so that the water can run in and ensuring all grit is removed. It may take several rinses to remove all the grit trapped between the layers. Remove hard outer leaves if necessary, then slice widthways across the leak, discarding the very tip of the leafy section and keep slicing down to the root at the white end. Season well and sote and butter until tender, or cook according to recipe instructions.