Duck Bigarade

Name: Duck Bigarade
Ingredients: 1 1/2 to 2 pounds duck breasts; Kosher salt; 1 tablespoon all-purpose flour; 1 cup Basic Duck Stock or chicken stock; Juice of 1 orange, preferably Seville (1/2 cup); 1 shot glass Grand Marnier or other orange liqueur, optional; 1 tablespoon cider vinegar or sherry vinegar; 1 teaspoon sugar; Grated zest of 1 orange; 1/2 sweet orange, quartered and thinly sliced