In a mixing bowl combine the olive oil, salt, pepper, rosemary and garlic. Whisk until well blended. Rub the marinade over the tenderloin. Let stand for about an hour. Grill the tenderloin to sear. Finish in a 325 degree oven until the internal temperature reaches 120 degrees. Cool immediately and thoroughly before slicing for sandwiches on freshly baked ciabatta bread, lettuce, tomato, onion and horseradish mayonnaise.