Combine first 5 ingredients in a small bowl; rub over roast.
Let stand at room temperature 30 minutes.
Bake roast at 450 for 45 minutes on lower rack of oven. Reduce temperature to 350, and bake roast 45 to 50 additional minutes or until a meat thermometer registers 145 (medium-rare) or 160 (medium).
Let roast stand 20 minutes.
Combine sour cream, lemon juice, and horseradish; serve with roast.