Rub the tuna loin with lime juice.
Mix the chopped herbs and spread them onto a sheet of cling film.
Roll the tuna in the herbs until it is completely coated.
Wrap the tuna tightly in cling film to form a cylinder and freeze for two hours.
Warm the sliced cucumber in a frying pan over low heat until excess water evaporates.
Add vinegar, sugar, and chilli, bring to a simmer, then remove from heat.
Pour the hot cucumber mixture over the sliced onion and allow to cool.
Stir in the mixed beans and parsley, then season with salt and pepper.
Slice the chilled tuna very thinly and arrange in a circle on a plate.
Garnish with mizuna leaves and the cucumber bean salad.