Preheat the barbecue to medium-high.
Set the prime rib, bone side down, in a 10" cast iron skillet or other barbecue friendly cooking vessel. Combine the mustards, garlic and rosemary in a small bowl. Brush on the roast; season with pepper. Set the skillet on one side of the barbecue. Close the lid and turn the heat off on the side of the barbecue the beef is sitting on; leave the other side set to medium-high. Cook the beef until the desired doneness is achieved (see Note).
When the beef is ready, transfer to a plate, tent with foil and rest 10 minutes before slicing. To make jus, remove excess fat from the skillet and then set on the hot side of the barbecue. Add the stock and bring to simmer. Pour into a gravy boat and serve with the beef.
Note: The best way to gauge doneness is to insert an instant read meat thermometer into the centre of the roast. A rare roast will be 125 degrees F, medium-rare will be 130 degrees F, and medium will be 135 degrees F to 140 degrees F. Remember that the meat will continue to cook when you let it rest before slicing.