Rub the outside of the roast with the mustard paste.
Set the Traeger to 250℉ and preheat, lid closed for 15 minutes using apple wood and hickory pellets.
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 reaches 125-130℉ for rare, or for medium-rare, 135℉. Do not overcook.
Remove to a cutting board and tent with foil for 20-30 minutes, slice and serve.
Notes
You can reheat prime rib in the oven. Cover with foil and reheat in a 300 degree oven for 20-30 minutes.