Well, I posted this a while back on the mahi-mahi thread but it works great for tuna, swordfish, shark or any other firm ocean fish....
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Here's a way I make it that is really awsome.
Light your charcoal grill and toss on a couple smallish pieces of firewood like oak or locust too.
Rinse fish pieces thoroughly in cold water and then squeeze to remove excess water and pat dry with paper towels. (wet fish don't cook very good for some reason)
In a small to medium pan melt 1/2 stick of real butter along with about two tablespoons of olive oil and one or two cloves of garlic. Heat slowly so the garlic just warms up and browns gently. Once browned, smash it up real good in the pan and remove from heat. (If you leave it on after being smashed, it'll burn the garlic)
Add about two heaping spoons of peach jelly and the juice from one lime. Home made jelly is best of course. Stir it together and start it back on some low heat. Chop up about a tablespoon of fresh cilantro and stir that in too.
This will be both a baste and a sauce for after the fish is done.
Brush a little olive oil on the fish and sprinkle with a some seasoning like a good cajun spice but go lightly as the baste will be the main seasoning. Even just some light salt and the baste is good.
When your coals are ready add a couple chunks of smoking wood like hickory, peach, apple etc and put the fish directly on the grill over direct heat. Cook a couple minutes before flipping. When you flip it, baste the cooked side with the sauce and close the lid a few minutes then recheck. Flip twice more with not much time on any side and baste each time with the sauce. It only takes about ten minutes total to cook. Less if thin or the fire is really hot. Just eyeball it and when it looks done, it probably is.
*note* I realize that it is considered improper by some people to cook tuna all the way through but I also know that some folks are turned off by rare fish. Actual cooking time or doneness is the prerogative of the cook.
Serve with some seasoned wild rice or similar and a glass of white wine and MMMMMmmmmmmmmmm-MMM! Your wife or girlfriend will think you're a master chef.