Set a large pot of water to boil; salt it. Leave the root end of the leeks intact (this will help keep them together), but split them almost to the root; trim off any hard green parts and wash the remaining parts well. Use string to tie the leeks into four bundles; this, too, will keep them from falling apart. Plunge them into the water and cook until very tender, about 20 minutes (when done, a thin-bladed knife will pierce them easily).

Drain the leeks and let them cool for about 5 minutes. Meanwhile, line a 14x3x3-inch terrine with enough plastic wrap to fold over the top.

Trim the root end from the leeks. If they are an inch thick or more, split them in half; if not, leave them whole. Make one layer of leeks with the white ends facing toward you; then one with the white ends away from you. Repeat, alternating directions, until all the leeks are used up. You can go about an inch over the top of the terrine, but no more.

Wrap the plastic over the top of the leeks, but do not fold the ends over the top; they must be left a little open to allow liquid to run out. Place a flat piece of wood or heavy cardboard that will fit snugly inside the mold on top of the leeks. Place the mold on a baking sheet with a lip, and invert it so that the wood or cardboard rests directly on two or three custard cups or ramekins; you want to elevate it a bit over the baking sheet so the liquid drains away from the terrine.

Refrigerate 24 hours. Invert the terrine so that it is right-side up. You will note that the leeks are packed almost as solid as a brick.

Invert the mold again, then unmold, but leave wrapped in the plastic; this will be much easier than you might imagine. Trim the ragged edges, then use a serrated knife to slice right through the plastic. Sprinkle with salt and pepper, then dress. The unused portion of the terrine will keep, well-wrapped and refrigerated, for several days.