how long to cook prime rib roast
1 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.irections. 1  Preheat oven to 450Â°F. 2  Pat the rib roast dry with a paper towel or napkin. 3  Smear the cut ends of the roast with the butter. 4  Place the roast (ribs down or fat side up) in roasting pan. 5  Sear the rib roast for 15 minutes at 450Â°F, then turn the oven to 325Â°F for the rest of the cooking time.