If the roast has a fat cap more than 1/4 inch thick, trim it with a sharp knife or ask your butcher to do it for you.
Tie the roast between the bones with butcher's string. (This discourages the eye of the meat from separating from the cap.)
In a small bowl, whisk together the mustard, Worcestershire sauce, garlic, thyme, and rosemary. If the dried rosemary needles are long, finely chop them before adding.
Slather the outside of the roast with the mustard paste.
Season generously with Traeger Prime Rib Rub.
Cook immediately, or refrigerate, uncovered, for up to 8 hours.
When ready to cook, start the Traeger on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 250F and preheat, lid closed, for 10 - 15 minutes.
Put the prime rib directly on the grill grate, fat-side up. Roast for 3-1/2 to 4 hours, or until the internal temperature of the meat (the tip of the temperature probe should be in the center of the meat) reaches 125 to 130F for rare, or for medium-rare, 135F. Do not overcook.
Transfer the meat to a cutting board - preferably one with a deep well so you don't lose the juices - and loosely tent the meat with foil. Allow it to rest for 20 minutes.
Snip the strings.
To carve, use a sharp knife or an electric knife to remove the rack of bone following the curvature of the meat. Then carve the meat across the grain into 1/2-inch-thick slices and arrange on a platter or plates. ?Serve with horseradish, if desired.