When the grill is ready to cook, remove the fish from the bag, dabbing off excess oil. Season and cook for barely 2 mins each side.
Put the oil into a food bag and add the tuna steaks. Rub well together and leave for 30 mins while you make the relish. Peel the cucumber, halve lengthways and scoop out the seeds. Chop the flesh into a small dice. Mix with the rest of the ingredients, seasoning to taste. Set aside.
To griddle: heat the pan to hot, then cook the steaks, turning after 2 mins, and cooking for another 2 mins each side depending on the thickness of the steaks. Meaty fish is best served slightly ‘pink’. Remove the steaks from the heat allow to stand for 3-5 mins, then spoon over the relish and serve.