Mix the mustards, Worcestershire sauce, and garlic in a small bowl, and brush generously all over the meat. Combine all the dried herbs and spices in a small bowl and sprinkle generously over the mustard. Allow to sit at room temperature while you prepare the Keg.
Set the temperature to 350°F (177°C), with the top control dialled to 1 and the bottom to 2. Place the prime rib on the smoker without a drip pan and let smoke for 4 hours. During this time, the temperature of the Keg should gradually fall to about 250°F (121°C). Test the internal temperature of the meat with a meat thermometer: it should reach 135°F (57°C) before resting and serving.
