Trim the leeks to about 7 inches long. Split them lengthwise to within 1 1/2 inches of the root end, leaving the root end intact. Submerge the leeks in a large bowl of water, let them soak to release any dirt or sand, for several minutes. Lift the leeks out of the water and discard the water.
Pour about 1-inch of water into a large saucepan, and set up a collapsible steamer inside. Bring the water to boil and steam the leeks, covered, until crisp-tender, about 6 minutes. Shock the leeks in a bowl of ice water and drain. Cut the leeks apart at the root ends and drain them on paper towels. Transfer the leeks to a shallow dish.
Brush the leeks with the vegetable oil and, working in batches, grill them over medium heat until charred and soft, about 4 minutes per side. (The leeks may be prepared up to the is point 1 day in advance and kept covered and chilled.)
In a bowl whisk together the herbs, bell pepper, olives, mustard, and vinegar. While whisking, add the oil in a stream until emulsified. Season with salt and pepper.
Pour the vinaigrette over the leeks and turn them to coat evenly. Marinate in the refrigerator for at least 3 hours or up to overnight.
When ready to serve, arrange the leeks on a platter and pour any remaining vinaigrette over the top. Garnish with red pepper, parsley, and egg.
Recipe courtesy of Gourmet magazine