Set the pork on a flat rack in a shallow roasting pan. Cover the end of each bone with a small piece of foil.
Make the marinade: Bring the apple cider to a boil over high heat and reduce by half. Remove from the heat, and whisk in the vinegar, mustard, brown sugar, garlic, thyme, and salt and pepper. Slowly whisk in the oil.
Using a pastry brush, apply the marinade to the roast, coating all surfaces. Cover it with plastic wrap and allow it to sit until the meat comes to room temperature, about 1 hour. (Or you can brush the marinade on the roast and return it, covered, to the refrigerator until ready to cook. Let the meat warm to room temperature before putting it on the grill.)
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 400F and preheat, lid closed, for 10 to 15 minutes.
Arrange the roasting pan with the pork on the grill grate. Roast for 30 minutes.
Reduce the heat to 325F. Loosely fill the crown with the stuffing, mounding it at the top. Cover the stuffing with foil. (Alternatively, you can bake the stuffing in a separate pan alongside the roast.)
Roast the pork for another 1-1/2 hours. Remove the foil from the stuffing and continue to roast until the internal temperature of the meat is 150F, about 30 minutes to an hour. Make sure the temperature probe doesn't touch bone or you will get a false reading.
Meanwhile, if desired, thread a sewing needle with sturdy thread and make a garland for the roast. (This is a good job for kids!)
Bring the roast to the kitchen and let rest for 15 minutes. Remove the foil covering the bones, but leave the butcher's string on the roast until ready to carve. Transfer to a warm platter. Garnish, if desired, with the cranberry garland and fresh herbs.
To serve, carve between the bones.