Preheat BBQ to low.
Cut steak in half lengthwise, then cut each piece in half crosswise. Season with salt and pepper and sugar. Place steaks in a cast iron skillet and place in the BBQ, cook until an internal temperature of 120 degrees. (3-5 minutes).
Take out of BBQ and turn heat on BBQ to high.
In a small bowl, combine butter, chives, mustard, and lemon zest.
Once BBQ has achieved at least 700 degree - sear steaks 30 seconds on each side.
Place steaks on dinner plates, cover with butter mixture and serve.