This recipe is part of our meal celebrating winter's tastiest fruit.
Pare peel from oranges and julienne it: You will need 1/2 cup for the sauce and 1 tablespoon to stuff in duck. Remove pith from oranges; cut into 1/4-inch wedges.
Bring vinegar, sugar, and 1 tablespoon water to a boil in a pan over high heat, taking care not to burn mixture. Cook, stirring, until thickened into a syrup, about 5 minutes. Add 1/2 cup chicken stock. Simmer 1 minute, stirring. Add remaining stock, port, and 1/2 cup orange peel. Simmer until reduced by a third, 8 to 10 minutes.
Preheat oven to 500°. Season duck cavity with salt and pepper; add 1 tablespoon orange peel, several sprigs of rosemary (or thyme), and 2 orange wedges. Place in a pan with a roasting rack, and cook until skin is golden brown, about 30 minutes.
Reheat sauce, and taste; if too sweet, add lemon juice. Remove from stove and swirl in butter to thicken.
Transfer duck to a platter; decorate with remaining orange wedges and herb sprigs. Serve with potato chips and sauce.