12 medium leeks (about 4 1/2 pounds), trimmed to about 7 inches, split lengthwise to within 1 1/2 inches of the root end, the roots trimmed but the root ends left intact, the tough outer leaves discarded, and the leeks washed well
vegetable oil for grilling
4 teaspoons Dijon-style mustard
3 tablespoons white-wine vinegar
1/2 cup extra-virgin olive oil
1/3 cup minced assorted fresh herbs, such as chives, parsley, mint, basil, and tarragon
1/4 cup finely diced red bell pepper
1/4 cup Kalamata or other brine-cured black olives, pitted and diced fine.