Heat two tablespoons of the oil in a large frying pan and preheat the grill. Fry the onions for 10 minutes until softened and golden, stirring occasionally. Sprinkle over the sugar and balsamic vinegar and then cook for another 2 minutes until the sugar has dissolved and is slightly syrupy, stirring continuously. Tip into a bowl and keep warm.
Return the pan to the heat and add the remaining tablespoon of oil. Season the steaks, add them to the pan and cook over a high heat for 3-4 minutes on each side for well done; or according to taste.
Split the ciabatta in half and then arrange on the grill rack cut-side up. Place under the grill until lightly toasted. Mix the mayonnaise in a small bowl with the two mustards.
Place a piece of the toasted ciabatta on each warmed serving plate. Add a smear of the mustard mayonnaise and then arrange a layer of the tomato slices. Season to taste and add the rocket. Place the steak on top and add the reserved caramelised onions. Drizzle over the rest of the mustard mayonnaise to serve.