Combine sour cream, mayonnaise, 3 Tbsp horseradish, 2 Tbsp mustard and 1 Tbsp parsley in bowl. Cover and refridgerate.
Preheat oven to 425 degrees.
Combine remaining 2 Tbsp horseradish and 2 Tbsp mustard with Worcestershire, rosemary, salt and peppercorns. Spread mixture over all sides of beef. Place beef in roasting pan. Roast until meat thermometer into center of beef registers 145 degrees, 50-60 minutes. Cover and let stand 10 minutes before slicing.
Remove string from meat and slice. Place on serving platter and sprinkle with remaining 1 Tbsp parsley. Serve with horseradish sauce. Makes 12 servings. Enjoy!