Do ahead: Destem the rosemary and mix with garlic, olive oil, and horseradish. If desired, process in a food processor to make smoother. Lay out two large sheets of plastic wrap in a cross. Place the roast on the plastic wrap, then slather on the garlic-horseradish sauce. Wrap tightly and refrigerate at least 3 hours or up to 24 hours.
Before baking, let the roast come to room temperature.
Preheat oven to 450 F. Bake for 15 minutes, then reduce the temperature to 325 F and continue roasting until internal temperature reaches 125 F for rare (about 1 1/4 hour) or 135 F for medium rare. (You don't want to over cook this cut, it'll get tough).
Optional but highly recommended: Make a gravy out of the drippings. Remove the roast from the pan and let rest before cutting it. In the meantime, pour the drippings into a medium saucepan.
Use some red wine or beef stock to deglaze the pan and get the crispy bits. Add those bits to the saucepan with the drippings. Sprinkle in about 2 tbs flour, whisking thoroughly to remove lumps. Add a cup or two of beef stock, then bring to a boil and let reduce for 1 to 2 minutes. Strain into a gravy dish.