To make the salsa roca, place the baby plum tomatoes in a dry, non-stick frying pan over a medium to high heat and blacken the skins (don’t worry if they burn a little as this gives the flavour). Remove from the pan, cool and roughly chop into a bowl.
Finely dice one half of the red onion and add to the tomatoes, along with the juice of 2 of the limes, frozen coriander, frozen chilli and 1 tbsp of the olive oil. Season with salt and pepper and stir well. Set aside.
Finely slice the other half of the red onion, place into a bowl and squeeze over the juice from the remaining lime. Set aside.
Drizzle the remaining olive oil over the tuna steaks and season with salt and pepper.
Place a griddle pan over a high heat (or have your barbecue ready with hot coals). Sear the tuna steaks for 1 minute on each side for a rare steak. Or, if you prefer, cook for an extra couple of minutes to give you a medium to well done steak.
Serve the tuna immediately with the salad, salsa and sliced red onion.