Pat 1 beef rump or round roast (about 3 pounds) completely dry with paper towels. Season all over with 2 teaspoons kosher salt and 1/4 teaspoon black pepper. Leave the roast at room temperature for at least 1 hour and up to 2 hours. While it rests, peel 1 pound medium carrots and 1 pound medium parsnips. Cut them in half crosswise, then slice each piece in half or quarters lengthwise so all the pieces are roughly the same size. SLice the brussel sprouts in half and set aside. Move an oven rack to the center position and preheat the oven to 375ºF.
Finely chop 3 garlic cloves. Strip the leaves from 3 sprigs of fresh rosemary and chop them finely (you should have 2 to 3 tablespoons). Stir the garlic, rosemary, and 1 tablespoon of the olive oil together in a small bowl. Rub this mixture all over the roast, then place the meat fat-side up in the center of a roasting pan or a rimmed baking sheet.
Scatter the carrots and parsnips around the roast. Drizzle with the remaining 2 tablespoons olive oil, sprinkle with the remaining 1/2 teaspoon kosher salt and a few grinds of black pepper, and toss so the vegetables are evenly coated. Spread them out into a single, even layer.
Roast until the outside of the beef is browned, about 30 minutes. Turn the oven down to 275ºF. Add the brussel sprouts to the vegetables in the pan and continue roasting until the meat is medium rare and an instant-read thermometer inserted in the thickest part reads 125ºF, about 1 hour more (start checking after 30 minutes). While the beef finishes cooking, prepare the horseradish sauce.
The roast is now ready to serve.