How to Make It
In a large saucepan, bring 2 cups of water to a boil over high heat. Add the leeks to the pan cut side down, cover and reduce the heat to low. Cook until the leeks are tender when pierced with a paring knife, about 15 minutes. Transfer the leeks to a paper towel–lined plate to drain, then arrange cut side up on a platter.
Meanwhile, bring a small saucepan of water to a boil. Fill a medium bowl with ice and water. Using a sharp paring knife, mark an X on the bottom of the tomato. Add it to the saucepan and blanch just until the skin starts to peel, about 30 seconds. Transfer the tomato to the ice bath to cool. Peel, halve and seed the tomato; cut it into 1/4-inch dice.
In a small bowl, toss the tomato with the olive oil, vinegar, mustard and Worcestershire sauce and season with salt and pepper. Spoon the tomato dressing over the leeks and serve.