No roast desiccates more easily than a big-breasted turkey—it’s a multidimensional anatomy problem. Turkey breast meat is finished cooking 10°F/5°C lower than the leg meat, so it is nearly impossible to get them perfectly cooked simultaneously. Miraculously, at least in its simplicity, the problem disappears when you slow-roast. By setting the oven temperature between the doneness temperatures of the breast and the leg, you cut your losses on both ends. The breast meat warms gently throughout, no section ever gets hotter than another, and the whole bird emerges moist and succulent.