Skirt Steak Fajitas with Lime and Black Pepper

Name: Skirt Steak Fajitas with Lime and Black Pepper
Ingredients: 2 large onions, peeled and cut lengthwise into 6 wedges, leaving root ends intact; 2 1/2 tablespoons olive oil; 2 1/2 teaspoons balsamic vinegar; 1 1/2 teaspoons salt; 1/4 cup fresh lime juice; 2 lb skirt steak, halved crosswise; 2 1/2 teaspoons coarsely ground black pepper; 18 flour tortillas; 1 cup fresh cilantro leaves; Accompaniment: tomatillo or tomato salsa; lime wedges; Special equipment: metal skewers or a grill basket