A day before roasting, remove the giblets and neck from the cavity of the bird and discard. If necessary pluck any stray pinfeathers off the duck with tweezers. Trim the neck flap and excess fat from around the cavity. Rinse and dry the bird well. Set the duck on a rack on a baking sheet, and refrigerate, uncovered, for 24 hours.
Heat the oven to 300 degrees F. Pierce the duck's skin all over (including the back), every 1/2-inch, with a skewer or small knife. Season the cavity with salt and pepper and stuff with 3 strips of the orange zest and the onion. Set the duck on a rack in a roasting pan, and pour a cup of water in the pan. Roast the bird for 3 hours, removing the duck from the oven every hour to prick the skin again.
Meanwhile, make the glaze: Combine the remaining orange zest, molasses, honey, coriander, pepper, orange juice, vinegar, and garlic in a small saucepan. Heat, stirring, over medium-high heat until warm. Remove glaze from the heat and set it aside at room temperature while the duck cooks.
Remove the duck from the oven and carefully, pour off the excess fat from the pan. (If desired reserve this fat for frying potatoes or wilting greens.) Raise the oven temperature to 450 degree F. Return the duck to the oven and roast until crisp and brown, about 30 minutes more.
Let the duck rest at room temperature for 10 minutes before carving. Brush the duck's skin with glaze 4 to 5 five times during the resting period. Carve the duck and transfer pieces to warm serving platter. Serve the remaining glaze at the table to drizzle over the duck, if desired.
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