Freeze the flank steak and soak 16 wooden skewers for 10 minutes each.
Slice the meat very thinly against the grain and thread onto skewers.
Heat grill to medium-high. Brush the steak with the vinegar and season with ¼ teaspoon each salt and pepper.
Grill until cooked through, 1 to 2 minutes per side. Serve with the blue cheese dressing and sprinkle with the scallions.