How to Make It
In a small skillet, toast the coriander, fennel and caraway seeds over moderate heat, tossing, until fragrant, about 2 minutes. Transfer to a spice grinder and let cool, then grind the toasted seeds to a powder. Transfer the spice mix to a small bowl and stir in the pimentón de la Vera, pepper and 2 teaspoons of salt.
Prick the duck all over with a sharp paring knife. Season the duck all over with the spice mixture and transfer it to a large plate. Refrigerate the duck uncovered overnight.
Preheat the oven to 300°. Rinse the duck under cool water and pat dry. Season the duck and cavity lightly with salt and stuff it with the onion, thyme and sage. Tie the legs together and transfer the duck breast side up to a rack set in a roasting pan.
Roast the duck for 1 hour, until an instant-read thermometer inserted in the inner thigh registers 140°. Increase the oven temperature to 425° and roast for 20 to 30 minutes longer, until the skin is crisp and an instant-read thermometer inserted in the inner thigh registers 160°. Tip any juices from the cavity into the roasting pan and transfer the duck to a carving board. Spoon off as much fat as possible from the pan juices. Carve the duck and serve with any pan juices.