01 - Preheat the oven to 375°F. Pat the duck dry using paper towels. Season both inside and outside with salt and pepper.
02 - Fill the duck cavity with quartered orange and fresh thyme sprigs. Tie the legs securely with kitchen twine.
03 - Place the duck breast-side up on a rack set in a roasting pan. Roast for 1 hour and 20 minutes, occasionally basting with pan juices.
04 - Increase oven temperature to 425°F and roast for an additional 20 minutes to achieve crispy skin. Verify doneness when juices run clear and internal temperature reaches 165°F in the thickest thigh area.
05 - Remove the duck from the oven and let it rest covered loosely with foil for 15 minutes before carving.
06 - Meanwhile, combine orange juice, zest, sugar, and white wine vinegar in a small saucepan. Simmer over medium heat until reduced by half, about 8 to 10 minutes.
07 - Add chicken stock and continue simmering for 5 minutes. Stir in Grand Marnier if using, then whisk in butter until silky and smooth. Season with salt and pepper to taste.
08 - Carve the duck and arrange on a platter. Drizzle with orange sauce and garnish with orange slices and thyme sprigs as desired.