As mentioned before, this is my go-to preparation for tuna steaks. It never disappoints!
Put a handful each of basil leaves and coriander leaves (and stems) together with 2 garlic cloves, half a small red chili pepper, juice of ½ lemon and some salt and pepper into a food processor and blitz with 3 to 4 tablespoons of extra virgin olive oil until well mixed and fully smooth. Put half of the marinate into a low serving dish, place the tuna steaks in the marinade and spoon over the rest, until completely coated. Cover the serving dish with clingfilm and let it infuse for about 30 minutes.
When ready, pre-heat a large frying pan or griddle pan on medium-high heat until very hot and place the tuna steaks into the pan (as the marinade has enough oil, there is no need to add extra) and sear them on both sides until you reach your favorite cuisson (I usually do 90 seconds for 2½ / 1″ thick steaks to only sear the outside).
This time, we served the (albacore) tuna steak with a tablespoon of the marinade, some tabouleh, and new baby potatoes. The marinade proved to be a great accompaniment to the boiled baby new potatoes; definitely, need to keep this in mind for a delicious summer potato salad!