Heat 2 tablespoons of oil in a frying pan. Sauté the duck breast only on the skin side. Then fry over medium heat for about 15 minutes. Peel and finely dice onion and potato.
Season vinegar with sugar, salt and pepper. Gradually fold in the rest of the oil. Add onion and potato cubes. Season with walnut oil. Lightly roast the walnuts in a coated pan without oil.
Remove the duck breast from the pan and let it rest for a short time. Clean and wash the lamb's lettuce, mix with half of the vinaigrette and arrange on plates. Cut the duck breast open and arrange on plates.
Drizzle with the rest of the vinaigrette and sprinkle with walnuts. Garnish with cranberries as desired.