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