Step 1

Preheat the oven to 325°. Prick the duck all over with a sharp knife. Season the cavity with salt and pepper and stuff it with the orange and lemon wedges.

Step 2

In a medium roasting pan, combine the water, coriander seeds and duck neck. Place the duck on a rack, season with salt and pepper, set it in the roasting pan and cover with foil. Bring the water to a boil over high heat. Transfer the duck to the oven and roast for 1 hour, until most of the fat has been rendered.

Step 3

Transfer the duck to a work surface. Increase the oven temperature to 350°. Strain the pan juices into a medium bowl and skim off the fat. Return the duck to the roasting pan and prick it all over a second time. Roast uncovered for 1 hour.

Step 4

Increase the oven temperature to 400°. Tip any juices from the cavity into the roasting pan and transfer the duck to a large rimmed baking sheet. Roast the duck for 45 minutes longer, until the meat is very tender and the skin is crisp.

Step 5

Meanwhile, set the roasting pan over moderately high heat. Add the orange and lemon juices and boil for 1 minute. Add the reserved pan juices and the soy sauce and boil for 1 minute longer. Pour the liquid into a small saucepan and bring to a simmer over moderate heat.

Step 6

In a medium bowl, make a paste with the butter and flour. Whisk in 1/4 cup of the hot liquid until smooth, then scrape the mixture into the saucepan. Simmer over low heat, whisking, until the sauce has thickened, about 2 minutes. Season with salt and pepper.

Step 7

Transfer the duck to a carving board and let rest for 10 minutes. Carve the duck and serve with the citrus sauce.