Step 1In a blade coffee grinder, grind the salt, peppercorns, juniper berries, allspice and orange zest to a fine powder.
Step 2Rinse the duck and pat it dry. Remove the neck, head, feet and wingtips if necessary. Liberally season the duck all over with the spice mix, rubbing to ensure it sticks. Place the duck in a 1-gallon sealable plastic bag and refrigerate for two days.
Step 3On the morning of the dinner, remove the duck from the bag, place it on a small platter and put it in the refrigerator, uncovered, to air-dry.
Step 4About three hours before dinner, heat the oven to 325 degrees. Use a carving fork or trussing needle to pierce the duck skin in several places at the base of both breasts and at the base of both thighs. Be very careful that you pierce the skin, but not the meat. Quarter an orange and stick it in the cavity.
Step 5Place the duck breast-side up on a rack in a heavy roasting pan and put it in the oven. Check on it from time to time and pour off any fat that collects in the pan. Roast until a thermometer inserted in the deepest part of the thigh reaches 170 degrees. This can take anywhere from 1 1/2 hours for a small 4-pound duck to about 2 1/2 hours for a big 5 1/2 pounder. If you find that your duck has cooked too quickly -- if it gets to 170 degrees before you're ready -- simply remove it from the oven and set it aside in a warm place.
Step 6While the duck is cooking, boil the turnips in salted water until they are just tender enough to pierce easily with a small knife, about 10 minutes. Drain and pat dry.
Step 7When the duck has reached 170 degrees, increase the oven heat to 450 degrees, add the turnips to the roasting pan and cook until the duck is well-browned and the temperature in the thigh is 180, about 20 minutes. Shake the pan every couple of minutes to roll the turnips around.
Step 8Remove the duck from the oven and set it aside in a warm place for 10 minutes before carving. Do not cover the duck or the skin will lose its crispness. Return the roasting pan with the turnips to the oven to finish browning.
Step 9Carve the duck and serve each piece with the turnips alongside.