Put the mustard in a small bowl. Whisk in the vinegar, salt and pepper. Slowly add the olive oil, whisking constantly, until completely incorporated. Stir in the shallots and tarragon. Set aside for at least 15 minutes.
Trim the asparagus, discarding woody ends. Steam, roast or grill the spears whole until just tender when pierced with a fork. Do not overcook.
Arrange the still-warm asparagus on a warm serving plate, spoon the dressing over it and turn the asparagus gently to coat.