Place the potato in a saucepan of cold salted water and bring to the boil. Cook for 20 minutes or until tender. Drain and rinse under cold water to cool.
To make the dressing, process the egg yolk, mustard, lemon juice and capers in a small food processor or blender for a few seconds. With the motor running, add the oil in a steady stream and process until combined. Stir through the dill.
To serve, place the potato, watercress and cucumber on serving plates, pour over the dressing and top with smoked salmon. Serves 6.