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Sear the rib roast (prime rib) for 15 minutes at the higher oven temperature (450 degrees F.), then turn the oven to the lower temperature (325 degrees F.) 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.rime Rib Roast (standing rib roast), at room temperature (very important) 2 tablespoons butter, room temperature. Preparation: Preheat oven to 450 degrees F. Pat the room-temperature standing rib roast (prime rib roast) dry with paper towels or napkins.
Note: For medium-rare prime rib, we want to take the roast out of the oven at 128Â°F, and it will continue cooking until it reaches 130Â°F. If you prefer a medium prime rib, take it out at 135Â°F with a target temperature of around 140Â°. Roast until the meat's internal temperature reaches 128Â°F, which will be another two and a half to five hours, depending on the size of your roast. 2  When the prime rib hits 128Â°F, take it out of the oven and transfer it to a cutting board and cover it with foil.