Place the beef in a roasting pan bone-side down and season it all over with the salt and pepper. Refrigerate, uncovered, for at least 12 and up to 48 hours. Allow the beef to come to room temperature for 2 to 3 hours before roasting.
Preheat the oven to 450˚F.
Roast the beef, bone side down, for 25 minutes. Decrease the heat to 300˚F and continue to roast until the center of the thickest part of the meat registers 115˚F on an instant-read thermometer for rare, or 125˚F for medium rare. After about 2 hours of roasting, begin checking for doneness, but it will likely take between 2 1/2 to 3 1/2 hours to roast. (The exact cooking time will depend on many factors, such as the weight of your roast, the temperature of the beef before it went in, and the functionality of your oven.)
Transfer the roast to a large cutting board and place rosemary sprigs underneath, on top, and all around it. Wrap the whole thing loosely with foil and allow the roast to rest for 20 to 30 minutes before carving.
For the horseradish cream: In a small bowl, whisk the crème fraîche or sour cream, horseradish, chives, lemon juice, and salt. Taste and add more lemon juice or salt, if desired.