PLACE meat in large glass dish; sprinkle evenly with pepper. Pour Italian dressing over meat; cover and marinate in refrigerator at least 1 hour.
PREHEAT oven to 325°F. Remove meat from marinade; discard marinade. Place meat, fat side up, in shallow roasting pan. Bake 2 to 2-1/2 hours or until meat thermometer inserted into thickest part of meat registers 130°F to 135°F for medium-rare doneness. Remove from oven. Cover and let stand 20 min. before carving.
MEANWHILE, mix 1 cup of the mayonnaise and the horseradish in small serving bowl. Mix remaining 1/2 cup mayonnaise, the vinaigrette dressing and thyme in separate small serving bowl. Serve both sauces with the meat.