how to cook a prime rib medium rare
1 The rib is cooked at 500 degrees F for exactly that many minutes. 2  For example, for a 6 pound roast: 6x5=30, so cooking time is 30 minutes. 3  Turn the oven off and wait 2 hours before opening the oven door. 4  Remove the prime rib and slice into the most perfectly medium-rare meat you've ever seen. Roast the 4-pound prime rib (see footnote if using a larger and smaller roast) in the preheated oven for 20 minutes. 2  Turn the oven off and, leaving the roast in the oven with the door closed, let the roast sit in the oven for 2 hours.