Trim tough, dark greens from leeks; discard. Halve leeks lengthwise, and cut off root ends, leaving as much of bulb intact as possible. Rinse each half under running water, carefully separating layers to loosen dirt.
Place a steamer basket over a saucepan filled with water; simmer. Prepare a large bowl of ice water.
Place leeks in steamer basket, cover and steam, until very tender, 10 to 12 minutes. Transfer to bowl of ice water to cool; drain. Dry on paper towels, cut side down, patting gently.
In a small bowl, whisk oil, mustards, and vinegar; season with salt and pepper. Arrange leeks on a platter; spoon vinaigrette over top.