Preheat oven to 180C. Drizzle beetroot with half the olive oil and wrap individually in foil. Roast on an oven tray until tender when pierced with a skewer (1¼-1½ hours). Unwrap and allow to cool.
Increase oven temperature to 220C. Place beef in a roasting tray, drizzle with remaining oil and season to taste, rubbing all over to coat evenly. Roast for 30 minutes, then reduce oven to 150C and roast until cooked to your liking, adding garlic in last 15 minutes of cooking (about 1¼ hours for medium rare; with an internal temperature of 65C). Remove beef from the oven, cover loosely with foil and set aside for at least 10 minutes to rest.
Meanwhile, when beetroot are cool enough to handle, peel them and pulse in a food processor until finely chopped. Season to taste and stir in horseradish and sour cream.
Cut beef into cutlets and serve with garlic and beetroot and horseradish, and pan juices.