Dice tuna into small cubes about 1 cm. Dice ginger and garlic. Finely chop coriander/cilantro. Slice spring onions. Mix all of the above together. Add a dash of sesame oil. Add olive oil to moisten. Add pepper to taste. When about to serve sprinkle with lemon salt. Do not add any acid like vinegar of lemon as it will cook the fish.
If you are able to get the Japanese ShiZu leaf. Then spoon some of the tuna onto a leaf. The flavour combination is lovely.