OFTEN, LEEKS ARE CALLED delicate, and they are milder than most onions, but really their flavor is rich, particularly in mature ones. And the flavor is different from that of an onion. Together, the two give depth to a broth, soup, stew. The blanched — white — bottom of the leek is the prime part, but the pale green gives color, and even the dark green leaves, if they’re not too old and tough, add to a liquid. They’re coarse, so afterward they’re discarded. The white and light green, thinly sliced, melt quickly when they’re sweated with a little oil or butter in a covered pot. Boiled whole leeks, especially younger, make an excellent salad. For a touch of raw onion flavor, I prefer chives or shallots, but you can use finely cut raw leek. However you use leeks, assume there’s grit hidden deep in the layers that must be carefully washed away. Often, you can see the color of the soil that was hilled up around them to blanch them.