Preheat a non stick pan until very hot. Place the baby tomatoes in and allow the skins to blacken. Don;t worry f they burn a little, this all add to the flavour.
Move around the pan until there is a lot of colour on them. Remove from the pan, allow to cool and roughly chop into a bowl.
Finely dice the onion (if you aren't keen on the strong flavour of raw onion, rinse it under cold water, then drain well) and add to the tomatoes along with the lime juice, coriander, chilli and olive oil.
Preheat a griddle pan (preferably a cast iron one) until very hot.
Drizzle a little more oil over the defrosted tuna steaks and season with salt and pepper.
Place the tuna on the griddle and leave for one minute each side for a rare steak, or 2-3 minutes each side for a well don esteak.
Serve with the peppery leaf salad and salsa roca immediately.