Score diamond shapes on the skin side of boned breasts of duck and put them in a marinade of olive oil, ground-nut oil and red wine. Leave for twenty-four hours in the refrigerator, turning them over and remixing the marinade after twelve hours.
The duck-breasts can be cooked either by grilling them over embers or in a cast-iron pot. Cook skin-side down, for about ten minutes. The duck produces its own fat when cooking. After ten minutes turn the duck-breasts over and cook for a further three minutes.
Remove them from the heat and keep warm between two plates in a pre-heated oven. In this way the duck flesh becomes tender.
Make a sauce by reducing wine vinegar with a chopped shallot over a low heat until it reaches the consistency of a syrup. Add Xérès vinegar and bring to the boil. Lower the heat and add butter in small pieces, whisking the contents until the sauce becomes creamy. Season with salt and freshly milled pepper. Add chives, chervil, parsley and tarragon. Keep the sauce warm in a bain-marie if it is not served immediately.
Cut the duck-breasts lengthwise in thin slices. Arrange in a fan shape around the individual warmed plates. In the centre of the plate, put potatoes which have been fried in duck fat. Cover the duck slices with herb butter and serve.