In a medium, shallow dish, combine the marinade ingredients, adding pepper to taste. Whisk until blended. Reserve ¾ cup in a small saucepan for basting. With a sharp knife, lightly score both sides of the steak in a crisscross pattern. Place the steak into the remaining marinade in the dish, turning to coat. Cover and marinate in the refrigerator at least 30 minutes, turning once.
Remove the steak and discard the marinade. Grill the steak, uncovered, over medium coals for 17 to 21 minutes, basting occasionally with the reserved marinade and turning once. Place the remaining basting marinade into a small saucepan on the hottest part of the grill and bring to a boil. Meanwhile, carve the steak into thin slices diagonally across the grain and arrange on a platter. Spoon hot marinade over the meat, and garnish with orange slices and rosemary sprigs.