1. Heat a large cast-iron skillet over medium-high heat. Sprinkle salt and pepper evenly over steak. Add steak to pan; cook 5 minutes on each side or until desired degree of doneness. Remove steak from pan; let stand 5 minutes. Cut steak diagonally across grain into thin slices.
2. While steak cooks, combine vinegar and next 4 ingredients in a medium bowl. Combine carrots and radishes in a medium bowl; add 1 tablespoon vinegar mixture, tossing to coat. Let vegetable mixture stand 5 minutes. Add steak to remaining 5 tablespoons vinegar mixture; toss well to coat.
3. Arrange steak on bottom half of bread; top with vegetable mixture and cilantro. Top with top half of bread; cut into 4 equal pieces.