You can make this two ways–everything in the blender, or chop the leek very fine and whisk everything together. The blender version will have a more pronounced leek flavor.
1 medium leek, white and light green parts only, chopped
6 T extra-virgin olive oil
2 T red wine vinegar
1 T Dijon mustard
1 t Worcestershire sauce
salt and pepper, to taste
1. Bring a small pot of water to the boil. Add leek and cook for a couple of minutes. Drain thoroughly and set aside.
2. Combine everything in a blender and buzz until the desired consistency is reached, or whisk everything together in a small bowl.