Put the duck legs on a baking tray, sprinkle with sea salt and black pepper, then put in the oven. Immediately turn the heat down to 180ºC/gas 4 and cook for about 35 minutes, or until the skin is crispy and the meat is moist and tender. Leave to rest for 5 minutes.
Destone and place the cherries in a salad bowl with the watercress. Crumble in the feta.
Make the vinaigrette by mixing together the ingredients in a jar or glass.
Shred the duck meat, add it to the bowl, pour over the vinaigrette, toss and serve.