The leek is a member of the onion family that has green leaves hiding a white stem with a sweet and subtle flavor. Because farmers mound earth around leeks to keep sun away from the leaves to make the stem white, leeks always have to be washed very carefully. This labor-intensive crop is usually expensive, so you need to choose leeks carefully, making sure you get more of the edible white than the throw-away green.
The slightly sulfurous aromatics in the leek give it medicinal properties. S-methyl cysteine sulfoxide lowers blood sugars; I even know one newly diagnosed diabetic who reversed his condition by eating enormous quantities of leeks. The sulfoxide in leeks is not as effective as medication, but it has a beneficial side effect that medication does not.
This sulfur compound is an antioxidant. Laboratory tests show that it stops the oxidation reactions that turn cholesterol into clogs. You probably cannot eat enough leeks to reverse atherosclerosis, but leeks can be an important part of a heart-healthy diet. The catch is, you have to eat them relatively fresh. A leek will keep for months in the fridge, but S-methyl cysteine sulfoxide dissipates after about the first twelve weeks. For best nutrition, eat leeks when they are harvested, in the fall and early winter.
You do not need to wash or trim leeks until you are ready to cook them. When you take them out of the vegetable bin, trim off the root end and remove any hard leaves. Cut a vertical slit through the center of the leek from bottom to top. Leaving most of the leaves on the leek at this point will keep it from falling apart when you wash it. Wash the two halves of the leek very carefully, removing all sand and grit. When you have washed the leeks, remove the rest of the leaves and slice.
Undercooked leeks are usually tough. You should always cook leeks until a thin-bladed knife passes through them easily. Leeks are often served with chicken, especially in Scottish cooking.