Preheat the oven to 425 degrees.

Crush the rosemary with the back of a knife to release the flavorful oils. Mix with oil, salt, and pepper.

Cut cross lines into the fat layer on the top of the roast. Rub on all sides of the roast. Let sit at room temperature for about 30 minutes.

Place roast in a roasting pan, bone side down. Prop if necessary.

Cook in hot oven for about 15 minutes, then reduce heat to 325° and cook until the internal temperature reaches 120° for rare (about 45 minutes to an hour). If you prefer it more done, temperature should be 130° for medium rare, 140° for medium, 150° for medium well and 160° for well done.

Allow the roast to rest for 15-20 minutes before cutting (you'll note the cut in this one - my husband couldn't wait). While it's resting, make the au jus.

Using the roasting pan with all the drippings, stir in the beef broth and stir to loosen all the bits at the bottom. Bring to a boil, stirring, then reduce heat to low and simmer until it is reduced by half. Mix the sour cream with the horseradish.

Serve with horseradish and au jus on the side.