In a large bowl, mix together the egg yolk, shallot, mustard and vinegar.
With a whisk, stir well.
Add the oil slowly while whisking continuously to emulsify.
Season with salt and pepper.
Set aside in the refrigerator.
Remove the green part from each leek (keep in the freezer to make broth, for example).
Remove the root and score the white part with a cross-shaped incision, starting at 2 to 3 cm (1 in.) from the base.
Wash the leeks thoroughly in cold water.
Place a steamer basket on the bottom of a Dutch oven.
Add water up to the base of the basket.
Cover and bring to a boil.
Steam the leeks for about 15 minutes or until tender.
Remove and let cool.
Place the warm or cold leeks on serving plates. Top with vinaigrette.
Garnish with capers, parsley and hard-boiled eggs.
Serve.
Note: If possible, prepare the vinaigrette the day before. This will allow the shallot to render all its flavour. Otherwise, refrigerate for at least 1 hour, whisking a few times.