Finely slice the tuna steak then finely chop and slice the other way until you have a good, evenly-chopped consistency. Scrape it into a bowl with half the coriander leaves and all the stalks.
In a separate bowl, mix the lime zest and juice with the ginger, sesame oil, extra virgin olive oil and soy sauce to make a delicious dressing. Have a taste, and tweak with extra lime, oil or soy until you’ve got a good balance.
Pour the dressing over your tuna and gently mix together – as soon as the acid from the limes hits the tuna it will begin to cook it so you only need to mix lightly.
Click off the stronger outer leaves of your little gem lettuces – you want about 16 in total. Wash them and spin dry well, then line them up on a board. The leaves are going to act as a vessel to hold your ceviche.
Divide the tuna ceviche between the lettuce leaves, sprinkle with a little fresh chilli, if using, the reserved coriander leaves, then serve straight away.