Cook the potatoes in a saucepan of salted boiling water for 12–15 minutes or until just tender.
Drain well and place in a large salad bowl.
Halve the avocado and remove the stone. Cut the flesh into pieces. Whisk together the dressing ingredients in a small bowl, then add to the warm potatoes. Mix in the avocado pieces, mustard and cress, lemon rind and rocket. Season well.
Divide the salad between 4 plates and serve.