Remove the duck from the fridge for half an hour before you’re ready to cook to get it to room temperature. Carefully and slowly score the skin of the duck breasts in a crisscross pattern without cutting through the fat into the meat. You will feel some resistance from the flesh against your knife if you’re cutting too deeply. Season duck with salt and pepper.
Arrange four plates with salad leaves. Set aside.
Use a heavy based frying pan and heat the oil. Place the duck breasts skin side down. The breasts should sizzle but shouldn’t be so hot that they burn. After 6-8 minutes of cooking pour off any fat that is collecting into a small bowl. Turn the breasts over and cook for a further 1-2 minutes. Remove from heat and rest for 2-3 minutes.
Slice breasts on the diagonal and place on top of salad leaves. Drizzle with cooking juices (skim off any fat first) and a few drops of vinegar for each breast. Top with some of the walnuts and cranberries.