Wash and peel the beetroots, then using a grater, coarsely grate them. If possible, wear gloves to stop the beetroot juice from staining your hands. Place the grated beetroots in a large mixing bowl.Add the cooked puy lentils and the watercress then mix well.To make the dressing, put all the ingredients in a jar and give it a good shake. Alternatively, if you don’t have a spare glass jar, you can put all the ingredients in a bowl or measuring jug and mix well using a fork.Add the dressing to the salad only once you’re ready to serve to stop the watercress from going soggy.