Rinse the duck with cold water and pat dry. Remove anything from the duck's cavity.
Cut the duck in half. First, remove the breast bone, then cut through the back of it. Remove any pieces of excess fat and cut off the tips of the duck wings.
Cut shallow slices into the breast skin side of the duck.
Place duck halves in a cast iron pan and rub with olive oil, shallots, rosemary, garlic, salt, and pepper. Cover with plastic wrap and put in the refrigerator for two hours.
Preheat Traeger Grill to 400°F. Put the cast iron pan into the grill. Make sure that the breast side is skin-side down in the pan. After 30 minutes, flip the duck pieces over to the other side and continue cooking until the internal temperature reaches 140°F. It will take about a 90 minutes of total cooking time.
Once the duck is done. Remove it from the grill and let it rest for about 5 minutes. Then serve.
To serve, remove the wings and the duck legs. Then remove the breast and slice it. Serve it with the optional Sweet and Sour Glaze.