Another simple way to serve Tuna steaks. This works better in a large frying pan rather than on the griddle, since the juices tend to bubble up during cooking.
First, mix up the marinade. Use a screw-top jar - the kind of thing that once held jam, or pickles. In the jar, put a teaspoon of brown sugar, a tablespoon of dark soy sauce, a tablespoon of Chinese or Japanese rice vinegar, and a tablespoon of sake (or dry sherry).
Put the top on the jar - carefully! - and shake hard until all of the sugar is dissolved.
Wash the Tuna steaks and place in a shallow dish just big enough to hold them. Pour the shaken marinade over the fish, then cover the dish with clingfilm and leave in the fridge for half an hour.
Heat a large non-stick frying pan with a teaspoon of vegetable oil (no more). Add a drop or two of sesame oil (optional). When the pan and oil are hot, slap in the tuna steaks. Cook on one side for about three minutes, adding a tablespoon or two of the marinade as the fish cooks.
Then turn the fish over, and cook for another two minutes, adding the remaining marinade.