Preparation

• In a medium bowl, whisk the lemon zest and juice, mustard, and 1/4 tsp. salt. In a slow, steady stream, whisk in the oil. Add the basil, parsley, capers, anchovies, and pepper flakes, and stir.
• Trim the dark-green tops from the leeks, leaving the white and light-green parts. Trim any scraggly roots but keep the root end intact. Halve the leeks lengthwise, then remove and discard any tough outer layers. Rinse the leeks well under cool running water. Fill a wide pot (8 quart works well) with 3 inches of well-salted water. Bring to a boil, add the leeks, and cook until just tender when pierced with a knife, 3 to 4 minutes. Drain and pat dry with paper towels.
• Prepare a medium-high (400°F to 475°F ) gas or charcoal grill, or heat a large grill pan over medium-high heat. Brush the leeks lightly with oil and season with salt and pepper. Grill, flipping once, until charred and tender, about 6 minutes total. Transfer to a platter and drizzle with some of the herb sauce. Serve warm or at room temperature with the remaining herb sauce and a sharp knife for cutting.