After browning the meat at 450 to 500 degrees Fahrenheit, cook the roast at 325 F for 13 to 15 minutes per pound for a perfect rare prime rib. However, most recipes recommend using a meat thermometer to accurately gauge when the roast is cooked rare.
To cook a prime rib roast, start by rinsing the exterior and trimming any excess fat. Next, rub the outside with the desired spices. Examples of tasty spice rubs include mixing black pepper, kosher salt, brown sugar, dried oregano and sweet paprika. Another option is a rub made of pepper, salt, minced fresh rosemary and minced garlic cloves.
Next, preheat the oven to the browning temperature for at least 30 minutes. Keep the prime rib out so it reaches room temperature. Place the meat in a roasting pan with the bone-side down. Cook the prime rib at this high temperature for 15 minutes or until the exterior is browned.
Reduce the heat, and insert a meat thermometer. Keep roasting the prime rib until the meat thermometer registers 110 F. Baste with the juices from the pan every 30 minutes. Remove the meat from the oven, and let the roast sit for 30 minutes to continue cooking. The finished internal temperature should be 115 to 120 F for a rare prime rib roast.