When it comes to cooking tuna steaks on a barbecue make sure that you do so on a very high heat. Also use the best quality steaks you can, as these will only require warming through to the middle.
In a bowl combine together well the lemon juice, olive oil, and 3 tablespoons of the cilantro leaves, garlic, shallots, cumin, cayenne pepper and salt.
Now into a large resealable bag you place the tuna steaks and then pour the sauce you have just made over them.
Make sure that you turn the bag over several times to ensure the sauce coats the steaks evenly then place in the refrigerator for 1 hour to allow them to marinate in the sauce.
About 15 minutes before you remove the tuna steaks from the refrigerator you should start up your barbecue so it is heated up correctly for cooking the steaks on it.
Just before you take the steaks out of the marinade after removing from the refrigerator lightly oil the grill on your barbecue and then place the tuna steaks on them.
Make sure the grill is placed as close to the heat source as possible as you need to cook the steaks on a high heat.
You should cook each side of the tuna steaks for around 4 to 5 minutes or until you feel they are cooked to the way people enjoy eating this type of fish.
Once they are cooked remove from the barbecue place on clean plates and sprinkle with the remaining cilantro and then serve.