Make the dressing. Combine the lemon rind, vinegar, tarragon, mustard and sugar in a small bowl and season to taste. Stir to mix, then gradually whisk in the oil. Alternatively, place all the ingredients in a screw-top jar and shake well to combine. Set aside.
Heat the oil (if used) in a large frying pan. Add the asparagus in a single layer and cook for about 5 minutes, turning occasionally. (The asparagus should be tender when pierced with the tip of a sharp knife and lightly patched with brown.)
Transfer the asparagus to a shallow dish and sprinkle with salt and pepper. Cover with the dressing, toss gently and leave to stand for 5 minutes.
Arrange the salad leaves in a serving dish, sprinkle over the onions and radishes and pile the asparagus in the centre of the leaves. Garnish with chopped herbs and thin strips of lemon rind. Serve on its own with bread or as an accompaniment to a main dish.