To make the mayonnaise, combine all ingredients in a processor and puree until creamy.
Place the beetroot and the water in a saucepan and bring to the boil. Reduce the heat to simmer, cover and cook for approx 30 minutes or until the beetroot is tender (but not too soft).
Drain and set aside until its cool enough to handle.
Peel the skin off the beetroot and cut it into quarters. Slice the beets into quarters and place on a bed of watercress. Top with mayonnaise dressing.
Note: Another delicious variation is substituting the sun dried tomatoes with 1 tsp drained capers and chopped dill and ½ tsp lemon juice.