1. Trim the tough dark-green ends from leeks; discard, or set aside for another use. Trim root hairs; discard. Then, starting about 1/2 inch from the root end (which stays intact), slice each leek lengthwise from base to tip with a sharp knife. Rinse leeks thoroughly to remove sand.

2. Fill a bowl with ice water; set aside. Bring a large pot of water to a boil. Salt well, and add leeks. Blanch until leeks are very tender, about 4 minutes. Transfer leeks to the ice bath; submerge them immediately. Once the leeks are cold, drain them, pat them dry, and set them aside.

3. In a small bowl, combine mustard and vinegar. Slowly drizzle oil into bowl, whisking to form a thick vinaigrette. Adjust seasoning with salt and pepper.

4. Lightly brush leeks with some vinaigrette, and arrange them on a medium-hot grill or grill pan. Grill leeks until they are hot and have grill marks, about 2 minutes per side. Remove leeks from grill, and toss with remaining vinaigrette. Serve.