Prepare the duck: Preheat your oven to 375°F (190°C). Pat the duck dry with paper towels and season it inside and out with salt, pepper, garlic powder, and thyme. Stuff the cavity with the sliced orange, onion, and rosemary sprigs.
Score the skin:
Use a sharp knife to score the skin of the duck in a crisscross pattern, being careful not to cut into the meat. This will help the fat render and crisp the skin.
Roast the duck:
Place the duck on a rack in a roasting pan, breast-side up. Roast for about 1.5 to 2 hours, or until the skin is golden brown and crispy and the internal temperature of the thickest part of the duck reaches 165°F (74°C). Baste occasionally with the cooking juices for extra crispness.
Prepare the orange glaze:
While the duck roasts, combine the orange juice, honey, soy sauce, balsamic vinegar, and orange zest in a saucepan over medium heat. Bring to a boil, stirring until the honey dissolves. Mix the cornstarch with water, then stir it into the glaze, letting it thicken for about 5 to 7 minutes.
Glaze the duck:
During the last 20 minutes of roasting, generously brush the duck with the orange glaze to caramelize. Repeat this process once more if desired for a glossy, sticky finish.
