Over high heat, briefly deglaze the pan with the vinegar, then add the stock and reduce by half. Lower the temperature and whisk in the remaining 2 tablespoons of butter. Cook for another 2 minutes, making sure the sauce does not boil or separate. Pour the sauce over the foie gras and the apples and serve. Depending on what is in season, you may substitute other cooking apples for the Golden Delicious.
Melfor vinegar is a honey vinegar that is common in Alsace.
Apple cider vinegar makes an acceptable substitution.