Using a sharp knife, segment 1 ½ of the grapefruits. Squeeze the juice of the remaining grapefruit half into a small saucepan and set aside for the dressing.
Place a large non-stick frying pan over a medium-high heat and add the oil. Sprinkle the duck with salt and pepper and add to the pan, skin side down. Fry for 6-8 minutes, making sure that the pan doesn't get too hot, until the fat is rendered and crisp. Turn over and fry for another 8-12 minutes until cooked through.
Remove from the pan, brush with the honey and set aside to rest.
Meanwhile, make the dressing: add the dressing ingredients to the grapefruit juice in the pan and place over a medium heat. Bring to the boil and simmer for 5 minutes, until reduced slightly. Remove from the heat and set aside to cool.
Slice the duck thinly and toss with the grapefruit and watercress. Pour the dressing over this to serve.
*Inspired by Donald T. featured in the Realfood Cookbook
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.