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.