I'm cooking a 21 lb prime rib and want it rare. How long should I roast it for (and at what oven temp)?

Is the bone on or off - check various sites for time temp you prefer - but- invest in a thermometer and pull the roast out at 120F ... it will continue to cook while resting. Rest tented for at least 20 minutes. I encourage horseradish.

For a roast that big, you'll probably want to cut in half -- if you use the Ann Seranne recipe Amanda shared above, you can follow the timing on the roasting chart for 11-12 pounds and roast them in the same oven.

Check out this segment from the MSL Show this week with Molly Stevens on roasting prime rib. http://www.marthastewart.com/873666/standing-rib-roast-prime-rib-yorkshire-pudding You can adapt the recipe to season as you prefer, but the guidelines for roasting are excellent. You can find a video link to the segment on the left margin of the page. Good Luck and Merry Christmas

Here's a great recipe for it with foolproof technique. http://www.food52.com/recipes/15387_ann_serannes_rib_roast_of_beef