what temperature do you cook a prime rib roast at
1 Place the roast (ribs down or fat side up) in roasting pan. 2  Sear the rib roast for 15 minutes at 450Â°F, then turn the oven to 325Â°F for the rest of the cooking time. 3  Every 1/2 hour, baste the cut ends of the roast with the fat accumulated in the roasting pan. Sear the rib roast for 15 minutes at 450Â°F, then turn the oven to 325Â°F for the rest of the cooking time. 2  Every 1/2 hour, baste the cut ends of the roast with the fat accumulated in the roasting pan.