how do you cook prime rib
4. Sear the prime rib at this temperature for 15 minutes. Then turn the oven to 325Â°F (162Â°C) for the rest of the cooking time. Every 1/2 hour, baste the cut ends of the roast with the fat accumulated in the roasting pan.Do not cover the roast.. Sear the prime rib at this temperature for 15 minutes. Then turn the oven to 325Â°F (162Â°C) for the rest of the cooking time. Every 1/2 hour, baste the cut ends of the roast with the fat accumulated in the roasting pan. Do not cover the roast.