• Thoroughly coat a roasting pan and rack with nonstick spray.
• Cut 4 slices from the center of the orange for garnish and refrigerate until ready to serve. Cut the remainder of the orange into wedges and set aside.

Prepare the duck:

• Remove the neck, giblets, liver and heart from the cavity of the duck and reserve for another use or discard. Trim away any flaps of excess skin from around the cavity and tail area. Rinse the duck in cool water and pat dry with paper towels inside and out.
• Season the duck's cavity with kosher salt and freshly ground black pepper, then stuff with the orange wedges and rosemary sprigs. Bend the wing tips under the bird and tie the legs together with cotton twine. Place the duck, breast side up on the prepared roasting rack.

  TIP: If you're able, prep the duck ahead of time and refrigerate uncovered overnight. This will help to dry the skin, making for a crispier end result.
• Before roasting, set the duck aside for 1 hour to allow it to come to room temperature (do not cover).

Roast the duck:

• Preheat the oven to 350°F.
• Using the tip of a sharp knife, prick the skin of the duck all over (1/4-inch spacing), being careful not to pierce all the way through to the meat. Rub kosher salt and pepper all over the duck, top and bottom, then place it in the oven and roast for 30 minutes.
• Remove the duck from the oven, turn it breast side down on the rack (see notes), rotate the roasting pan and continue roasting.
• Repeat this process every 30 minutes until an instant-read thermometer inserted in the thickest part of the duck's thigh registers 165°F. Total cooking time should be about 2-1/2 hours.
• Once cooked, tilt the duck to drain the fat and juices that have accumulated in the cavity into the roasting pan. Tent loosely with foil and allow it to rest for 10 to 15 minutes before carving.

Make the orange sauce:

• While the duck rests, prepare the sauce. Combine the orange juice, rosemary and Dijon mustard in a small saucepan and bring to a rapid simmer over medium-high heat. Cook, uncovered, until reduced to about 1/2 cup, 6 to 8 minutes.
• Remove from the heat, add the vinegar, butter and orange zest and stir until the butter has melted. Season to taste with salt and pepper.

Carve and serve:

• To serve, transfer the carved duck to a serving platter and garnish with fresh orange slices and sprigs of rosemary. Drizzle with sauce and serve immediately.