1Place beef on rack in shallow roasting pan. In small bowl, mix garlic, salt, pepper, rosemary and oil; rub mixture over roast. Let stand at room temperature 30 minutes.
2Heat oven to 450°F. Insert ovenproof meat thermometer so tip is in thickest part of beef and does not touch bone. Place pan on lower oven rack. Roast uncovered 45 minutes. Reduce oven temperature to 350°F. Roast 45 to 50 minutes longer or until of desired doneness (140°F for medium-rare, 155°F for medium). Cover loosely with foil; let stand 15 minutes.
3In small bowl, mix sour cream, lemon juice and horseradish. Carve beef; serve with horseradish sauce.