Mix together the tuna, avocado, tomato, chilli and watercress in a non-metallic dish and set aside for 10 minutes.
Meanwhile, heat a large frying pan. Place one tortilla on a chopping board. Layer the mozzarella, spring onions and coriander on top of the tortilla. Put the other tortilla on top, press down a little and transfer to the frying pan. Cook for 1 minute, then turn over and cook for a further minute. Cut into wedges and serve with the tuna salsa.