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