How to Make It
Stir together balsamic vinegar, soy sauce, and a pinch of salt in a large bowl. Add steak, and turn to ensure steak is evenly coated in marinade. Cover and refrigerate at least 2 hours or up to 8 hours.
Preheat a grill to medium-high (400°F to 450°F), or heat a grill pan over medium-high. Toss together bell peppers, onion slices, olive oil, 1 teaspoon salt, and 1/2 teaspoon black pepper in a medium bowl. Place bell pepper mixture on oiled grates, and grill, uncovered, turning occasionally, until charred and softened, 10 to 12 minutes. Immediately return mixture to bowl, and toss with cilantro, chives, lemon juice, and garlic. Cover with plastic wrap, and let steam 15 minutes.
Remove steak from marinade; discard marinade. Wipe excess marinade off steak. Place steak on oiled grates, and grill, uncovered, turning occasionally, until lightly charred on both sides and medium-rare, 10 to 15 minutes. Transfer to a cutting board; let rest 15 minutes.
Thinly slice steak against the grain; season with remaining 1/2 teaspoon salt and remaining 1/2 teaspoon black pepper. Serve steak with charred pepper mixture and grilled ciabatta.