Preheat a barbecue flat-plate or chargrill pan to hot. Brush steaks with 1 tbsp olive oil, season with salt and pepper and cook for 2 – 3 minutes per side or until cooked to your liking. Remove steaks from heat and cover loosely with foil for 5 minutes.
While steaks are resting decrease heat to medium and cook onion with remaining olive oil on one side of the barbecue flat-plate, stirring, for 5 to 6 minutes or until softened and lightly golden. Toast ciabatta on the other side of the barbecue until toasted.
Trim cooked steaks of any excess fat and slice steaks thinly against the grain.
Spread relish across bread bases and top with rocket, tomato, cheese, sliced steak, onions and bread tops. Serve with a salad.