Thinly slice the onion.

Spray a frying pan with oil spray and heat it over medium heat.

Add the sliced onion and fry for 3 minutes until beginning to soften.

Season the onion with the salt and sugar.

Cook the onion for 10 minutes, adding the water gradually when the pan becomes dry.

Add the balsamic vinegar and cook the onion for 5 minutes until caramelised.

Spray the steak with oil spray and season it with Cajun seasoning, salt, and black pepper.

Heat a clean frying pan over high heat and add the seasoned steak.

Fry the steak for 2 to 3 minutes on the first side.

Turn the steak and fry it for 2 to 3 minutes on the second side for medium-rare.

Transfer the steak to the cutting board and rest it for 4 minutes.

Mix the light mayo and Dijon mustard in a bowl.

Stir in the remaining water, a little at a time, until the Dijon mayo is spreadable.

Toast the ciabatta rolls until lightly crisp.

Slice the rested steak thinly against the grain.

Spread the Dijon mayo inside each toasted ciabatta roll.

Layer the rocket and sliced steak in each ciabatta roll.

Top each sandwich with caramelised onions and drizzle with balsamic glaze if using.