The cooking method for this show-stopping roast may seem unconventional, but that's by design: Roasting at an initial high temperature sears the outside into a crust, then shutting off the oven and leaving the roast inside allows it to cook itself to the perfect medium rare. Use that time to whip up the creamy horseradish sauce.
For the prime rib: Preheat the oven to 500 degrees F. Put the butter, horseradish, peppercorns and 2 tablespoons salt in a medium bowl and stir until evenly combined. Slather the seasoned butter evenly over the top and sides of the roast (leave the bottom bare) and put on a rack set in a large roasting pan. Roast for 35 to 40 minutes (5 minutes per pound), then turn the oven off and let the roast sit inside the oven for 1 1/2 hours (do not open the oven door during this time). It should be dark golden brown on the outside and medium rare on the inside.
For the sauce: Meanwhile, whisk the sour cream, horseradish, 2 teaspoons salt and 1 teaspoon pepper together in a medium bowl until well combined. Stir in the chives, cover and refrigerate until ready to serve.
Cut the roast into 1-inch-thick slices and serve with the sauce.
To help plan your meal, decide what time you want to serve the meal and count back about 2 hours--that's when the roast should go into the oven.
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