2. Bring a medium sized pot of water to a boil and prepare a bowl of ice water. Slice the leeks lengthwise, put the flat side down and thinly slice them across. Rinse in cool water and drain well. Blanch the leeks in the boiling water for 1 minute, drain and plunge them into the ice water. Drain again and store the leeks in the refrigerator.
3. Combine the vinegar, shallots, sugar, mustard and chervil in a bowl. Whisk in the olive oil a little at a time until the vinaigrette is emulsified. Season with salt and pepper. Squeeze out the excess liquid from the cucumber and leeks and combine them with the vinaigrette.
4. Bring a large pot of water with a tablespoon of salt to a boil and prepare a large bowl of ice water. Add the asparagus to the boiling water and cook for approximately 5 minutes or until you can bend the spears without breaking them. Immediately plunge the asparagus into the ice water. Drain and pile the asparagus on a long plate with the leek and cucumber vinaigrette spooned over the top. Serve at room temperature.