Heat half the oil in a non-stick frypan and cook onion over medium heat until softened. Add sugar and cook for 1 minute before adding balsamic vinegar, cook for a further 1-2 minutes, until soft and sticky.
Combine mayonnaise, lemon, garlic and parsley in a bowl to make a herbed aioli. Season steaks and coat with remaining oil, barbecue for 2 minutes on each side or until cooked to your liking.
Spread 4 slices toasted bread with aioli. Top with rocket, tomato, bacon, steaks, cooked onion and cheese. Top with remaining bread and serve with fries.