Chill the tuna in the fridge before slicing.
Slice the tuna loin into 0.5 cm cubes and transfer to a bowl. Return it to the fridge.
Peel and dice the avocado into 0.5 cm pieces.
Finely chop the small red onion and the chilli, if using.
In a separate bowl, whisk together the zest and juice of the lime, soy sauce, grated ginger and sesame oil.
Remove the tuna from the fridge, add the avocado, onion and chilli, then pour over the dressing and gently toss to combine.
Taste and adjust with a little salt or extra lime juice if needed.
Cover with cling film and chill for 15–20 minutes to allow the flavours to meld.
To serve, press the mixture into a ring mould, lift off the mould and sprinkle with black sesame seeds.