Bring 2 quarts water to boil over medium-high heat in stock pot or large pan with a steamer basket insert.
In a small bowl, mix together vinegar, salt, mustard, and oil with a small whisk or fork until combined well. Gently stir in capers. Set aside.
When water is at a rolling boil add leeks to steamer pan and steam for about 10 minutes until tender. Remove with tongs or a slotted spoon and lay on a platter or serving plate and cover with vinaigrette while hot.
Leek Preparation Information:
Wash under water to remove visible dirt. Cut off the roots of the leeks. Slice leeks lengthwise. The white and light green parts are the mildest, the darker green parts are stronger and slightly bitter. The darker parts are best used in making stock. Make crosswise cuts along the leek that you intend to use. Place the chopped leeks in a bowl filled with cold water. Using your hands, agitate the leeks to dislodge dirt or sand. Use a slotted spoon to remove the leeks. Discard the water and repeat until the leeks are clean.