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 Dijon 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 and season generously with Traeger Prime Rib Rub on all sides. Cook immediately, or refrigerate, uncovered, for up to 8 hours.
When ready to cook, start the Traeger according to grill instructions. Set the temperature to 250 degrees F 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 130 degrees F for rare, or for medium-rare, 135 degrees F. Do not overcook.
Transfer 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 meat to rest for 20 minutes. Snip the strings.
To carve, use a sharp knife to remove the rack of bone following the curvature of the meat. Carve the meat across the grain into 1/2-inch thick slices. Serve with horseradish, if desired. Enjoy!
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