Raymond.....isn't that salmon? lol.
I have my patented tuna marinade for this purpose. Get some olive oil, garlic, black pepper, tomato puree, chilli powder and half a lime.
Squeeze the lime juice into a large flat dish that your two tuna steaks will fit into. Add the garlic (to taste....I go for about 1 glove of finely chopped), a few grinds of fresh pepper, chilli powder as desired (I don't use a lot...maybe a teaspoon full) a tablespoon of tomato puree and two tablespoons of olive oil. Blend it all together nicely using a spoon then place your tuna steaks in it, being sure to coat both sides well.
Cover it with clingfilm an whack it in the fridge for a few hours, turning the steaks over now and again so both sides get a good coating of the mixture.
Before you come to cook them, allow them to warm to room temperature. This stops your fish being too raw in the middle. Grill it for a few mins either side, you can tell when its done by looking at the side of the fish as it goes slightly grey. Too pink and it's floppy, too grey and its dry.
Serve with a nice crisp salad and some pasta, using the leftover marinade as your pasta sauce.