Dice the avocados, mango and cucumber and mix with chilli, coriander, salt, pepper and sesame oil.
Toast the pine nuts with a little sea salt in a hot, dry skillet.
Season the tuna with salt and black pepper and roll in the sesame seeds. Sear the tuna briefly in a little oil in a hot frying pan so that it has browned all over (but it will still be completely raw inside).
Slice the fish in thin slices and put it together on a plate with the salad. Mix together the soy sauce and lemon juice and spoon it over the fish.