Light the BBQ and get it hot. Alternatively, pre-heat a griddle pan.
Add all the chimichurri sauce ingredients to a food processor and pulse until finely chopped. Adjust the seasoning and add more chilli if you like.
Rub the tuna steaks with the olive oil and season with salt. Barbecue the tuna steaks (or cook in the griddle pan) for 1–2 minutes on each side, adding more or less time depending on how pink you like it. Add the limes cut side down onto the barbecue grill or griddle pan and chargrill until lines appear.
Serve the tuna with a generous spoon of the chimichurri sauce and a lime half.