To make the Teriyaki Sauce – first place the mirin, soy sauce and castor sugar in a small saucepan and bring to boil.  Remove from the heat and leave aside to cool and then stir in the ginger.

Rub the tuna on both sides with some salt and crushed pepper corns and chopped coriander, leave to set for 1 hour, place the teriyaki sauce on top of the fish and then leave to marinade for another 2 hours.

Heat the olive oil in a non stick pan until smoking hot.  Remove the tuna from the teriyaki marinade and place in the hot pan.  Cook for around 1 minute on each side and then pour over the teriyaki sauce on the fish and cook for a further minute.  Remove the tuna and allow the sauce to thicken.  Place the steaks on the plates, spoon over each of them some sauce, and serve with asparagus spears and stir fried red sweet pepper.