1. Finely chop the tuna and keep in a bowl on ice.
2. For the soy glaze, heat the soy, vinegar, sugar and water, then infuse for 5 mins with the ginger, lemongrass, bonito, and half the wakame, and the stems from the coriander.
Pass, thicken with a little arrowroot, then pass again.
3. Mix the tuna with 3 tbsp of the soy glaze, 1 tbsp cashew nuts, a pinch of salt and the juice and zest of ¼ lime.
4. Dress the bok choy leaves with some soy glaze.
5. Rehydrate the remaining seaweed in hot water, then chill.
6. Serve the tuna ice cold on chilled plates; arrange the coriander leaves, dressed bok choy, pickled ginger, seaweed and nuts over and around. Spoon over a little more of the glaze to finish.