Meanwhile, carefully split flank steak in half horizontally as if opening a book, cutting into 2 thin pieces. Slice each piece in half (for a total of 4 pieces). Season with ancho chile powder, salt and pepper. Grill steak to taste, about 5 minutes, turning once, for medium-rare. Transfer to a platter to rest. Add scallions to grill; grill 6 minutes, turning once, until charred.