Mix together the balsamic vinegar, garlic, oregano and chilli in a shallow dish. Add the steaks and leave to marinate for up to 2 hours in the refrigerator.
To prepare the tomato salsa: gently heat the olive oil in a frying pan. Add the onion, garlic and chilli. Gently cook for 5 minutes until the onion is soft and golden.
Remove the steaks from the marinade and season with salt and pepper on each side. Add the marinade to the pan along with the tomatoes and cook on a high heat for 3-4 minutes until the tomatoes start to burst in the pan. Throw in the rocket to wilt, reserving some for the sandwich.
Cook the steaks on the barbecue or grill for 1 minute on each side. Slice the steaks and set aside.
Place the reserved rocket into a large bowl with a splash of olive oil and balsamic vinegar. Season and toss the leaves around the bowl.
Slice and oil the bread and place onto the barbecue or griddle, oiled side down for a few seconds to toast. Remove, rub with garlic and place onto a plate.
To assemble the sandwich: place the tomato salsa and wilted rocket onto the base of the bread. Place some of the steak slices on top. Spoon over more of the tomato salsa, gently squashing the mixture down. Top with the remaining beef and the other slice of ciabatta.
Cut each sandwich in half on the diagonal and serve immediately.