Combine all ingredients in a blender and puree until creamy. Refrigerated, this mayonnaise will keep for 2 days. If the mixture separates stir well to combine.
Cut the greens from the beets leaving 1 inch of stalk connected to the beet. Scrub the beets, being careful not to puncture the skin.
Place in a 1.5 litre saucepan with water to cover. Bring to boil, reduce heat, and simmer for 35 minutes or until soft.
Drain beets and allow to slightly cool. Slip skin off with your fingers under running cold water. Wash hands immediately after peeling to remove stains from hands.
Slice the beets into quarters or rounds and place on a bed of watercress. Top with mayonnaise dressing.