Preheat the oven to 220掳C (425掳F). Prick every inch or so (2.5 cm) of the duck skin with a fork or tip of a sharp knife (this helps fat to render and results in crisper skin). Tie the legs together; fold and tuck the wings under the body. Season with salt and pepper. Place breast-side up on a rack in a roasting pan. Fill the pan with hot water to just below the duck. Cover duck with foil. Roast 1 hr 30 min.
Step 2Meanwhile, make the glaze by mixing the thawed orange juice concentrate and vinegar. After the duck has roasted 1 hr 30 min., reduce oven temperature to 160掳C (325掳F). Uncover duck and brush with about half the glaze. Roast 10 minutes more, then brush with remaining glaze. Roast additional 10 to 15 min., or until skin is crispy.
Step 3Also, while duck is roasting, make the orange sauce. Melt the butter and brown sugar in a saucepan over medium heat. Slowly and carefully, watching for splatters, whisk in zest, orange juice, chicken stock, lemon juice, soy sauce and brandy. Bring to a simmer and reduce by one-third. Mix cornstarch with 3 tbsp (45 mL) cold water until smooth. Whisk into sauce; cook until thickened. Keep sauce warm.
Step 4Rest duck 10 min. before carving. Garnish platter with orange slices and parsley, if desired. Serve orange sauce alongside.