Fresh tuna has a meaty texture and a taste that's completely different from the canned variety (although tuna canned in olive oil is a delicacy in its own right). It is delicious served raw (in sushi or sashimi) or lightly seared, so it's still pink and moist inside.
If you like, serve this as an appetiser, instead of a main. This will serve two as a main, four as an appetiser.
Choose a piece of tuna that's about 2.5cm (1in) thick. It has the best taste and texture if it's lightly seared so it's almost raw, and still a deep pink inside, but if you like it more cooked, leave it in the pan for longer.
Make the sauce first. Put the lemon juice, soy sauce and rice vinegar in a small bowl and mix. Taste for seasonings and correct, if necessary. Grate the lemon zest over the bowl and mix well, then set aside.
Finely mince the spring onion.
Put the sesame seeds in a small, unoiled skillet and place it over a low flame. Shake the pan almost constantly until the sesame seeds are fragrant and lightly toasted. As soon as the sesame seeds are toasted, pour them into a small dish.
Heat a skillet (preferably plain cast iron) over a medium-high flame until it's very hot.
Brush the tuna on all sides with cooking oil, then place it in the hot skillet. For rare, cook it for about 30 seconds in total, flipping it over halfway through. If you like the tuna more done, cover the skillet as it cooks, and let it sear for a little longer.
Take the tuna from the pan and place it on a cutting board and let it rest briefly. Slice it about 8mm (⅜in) thick and put the pieces on plates.
Spoon some of the sauce over each portion, then sprinkle with spring onions and sesame seed. Pour the sauce into individual bowls and serve immediately with steamed rice and Japanese pickles.