Trim the rib roast, removing all of the fat cap and the “tail”. The tail is more fat than muscle, so I just take the whole thing off and then trim the meat out of it for stew or stir fry meat.
Dry brine the roast using kosher salt. Kosher salt is better for this, because it is designed for preparing meat. Sprinkle it on all sides of the roast, wrap the roast in clear plastic wrap and put it in the fridge for at least 6 hours. The salt will draw moisture out of the roast, but as it sits the roast will reabsorb the moisture and carry the salt in with it.
While the roast is dry brining, mix up the paste rub. The herbs are fat soluble and mixing it up ahead allows the flavors from the herbs to spread through the oil. Start with ¼ cup of oil and add more if needed.
Preheat the cooker or grill. Remove the roast from the fridge and unwrap it. Apply all of the paste rub on all sides of the roast. Cook the roast with medium, indirect heat.
While the roast is cooking, prepare the au jus and the horseradish sauce.
When the roast reaches 130 degrees (Medium Rare) to 140 degrees (Medium) internal temperature (2.5-3 hours on a Pit Barrel Cooker). Put the au jus mix in the bottom of a roaster pan, and put the prime rib in the roaster at 150-170 degrees to let it rest; juices from the roast will mix with the au jus.
Before serving, pour the au jus out of the roaster pan to serve with the roast along with the horseradish sauce.
