what temperature is medium rare prime rib
1 After 20 minutes turn the oven down to 325 degrees F. and roast until the desired internal temperature is reached (see guide below). 2  For medium-rare this will take approximately 15 minutes per pound. 3  Transfer to a large platter, and let the prime rib rest, loosely covered with foil for 30 minutes before serving. When the oven is hot, put the roast in and cook for 20 minutes to sear the outside of the roast. 2  After 20 minutes turn the oven down to 325 degrees F. and roast until the desired internal temperature is reached (see guide below). 3  For medium-rare this will take approximately 15 minutes per pound.