I usually cook it on a really hot Griddle pan, or stir fry it.
One awesome recipe I use is
1 Tuna Steak (get good ones, more than 1" thick, and really deep red in colour)
Teriyaki Sauce (or make your own, with a bit of light soy sauce, a little bit of dark soy sauce and some Merrin, mess around with the formula, make it to your own Taste!)
1.Rub the marinate into the Steak, on both sides
2. get your griddle pan out, put a little groundnut oil in it, to prevent the steaks sticking!
for reference, this is a griddle pan!
3. Get the pan mad hot!
4. Chuck the steak on and let it start cooking. It will begin to turn white.
5. Once the whiteness has reached halfway through the steak, turn it over.
6. ONce the side of the steak is white completely, cut it open, it should be almost cooked, with a pink band in the centre. Dont worry, you wont get food poisoning from this! If you cook the thing the whole way through your steak will be tasteless!
Serve with some roasted peppers, red onions, potatos in a chilli marinate etc, tastes beautiful!
My other fish favourites are...
Tuna Chunks with Cous Cous
Smoked Mackeral fillets and hardboiled eggs
Sardines and Chopped Tomatos