PRIME RIB: Take prime rib out of the refridgerator atleast 2 - 3 hours before cooking to bring to room temperature. Pat dry all over with paper towel.
PREHEAT: STANDARD oven to 460˚F (240°C) | FAN/CONVECTION 430˚F (220°C).
GARLIC BUTTER: Mix together the butter and garlic and 1/2 tablespoon of salt in a small bowl until combined.
SKILLET OR ROASTING PAN: Arrange garlic halves in a heavy-based pan or cast-iron skillet. Spread 2 tablespoons of garlic butter on the underside of the beef (bone-side) to completely coat. Place the roast, bone-side down, in the pan/skillet on top of the onion and garlic halves (the bones create a natural roasting rack for the meat). Spread another 2-3 tablespoons of garlic butter on the top and sides, rubbing into the flesh. Coat well.Season with remaining salt and pepper.
ROAST: For 20-30 minutes or until the roast has nicely browned.
GARLIC BUTTER 2: Carefully remove roast from oven. Spread with remaining garlic butter.
REDUCE OVEN TEMPERATURE: STANDARD oven 250°F (120°C) | FAN/CONVECTION oven 220°F (100°C).
COVER WITH FOIL and continue to slow roast for 1 - 1 1/2 hours, basting every 20-30 minutes with the pan juices, until reaching your desired preference. Internal temperature should read 118°F | 48°C for med/rare. (TIP: Start checking internal temperature after 45 minutes of roasting. Continue to keep checking with each baste to ensure it doesn't over cook.)
Once cooked to your liking, allow to rest for 20 minutes. The roast will continue to cook as the juices inside settle, raising the internal temperature to 130°F | 55°C for a perfect medium-rare prime rib.
REST: Transfer roast to serving plate, tent loosely with foil and let rest for 20 - 30 minutes. Internal temperature will rise to 125°F (52°C) for medium rare.
SLICE AND SERVE with Red Wine Jus/Sauce.
RED WINE JUS: Place skillet with pan juices and the cooked garlic halves left in the pan on the stove top over high heat. Stir in 2 1/2 cups of the beef stock and all of the wine. Bring to a fast simmer for 10 minutes until iquid has reduced by roughly half.
REDUCE HEAT to medium. Whisk cornstarch (cornflour) into the remaining beef stock. Whisk half of the mixture into the red wine jus. Sauce will begin to thicken. Whisk in remaining cornstarch mixture for a thicker jus. Strain jus into a bowl, pressing garlic juices through the strainer, then pour jus into serving jug.