Heat the oil (if used) in a large, nonstick frying pan and add the asparagus in a single layer. 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. Remove from the pan to a shallow dish and sprinkle with salt and pepper. Cover with the tarragon and lemon dressing, toss gently and leave to stand for 5 minutes.
Arrange the rocket on a serving plate. Sprinkle the spring onions and radishes over the rocket. Arrange the asparagus in a pile in the centre of the rocket. Garnish with herbs and lemon rind. Serve the salad on its own with bread or as an accompaniment to a main dish.