Using a spiralizer, turn the white radish into noodles, then plunge them into a bowl of iced water for 10 - 15 minutes until they are crisp and very cold.
For the dressing, whisk the soy sauce, rice wine vinegar, wasabi and honey, olive oil and sesame oil together in a small bowl. Set aside.
Heat a non-stick frying pan over a medium-high heat and then add a splash of oil. Season the tuna with salt and pepper and sear all over - just until the surface is a nice, toasty brown colour; make sure you don't overcook it. Remove from the pan and leave to rest for 2 - 3 minutes.
Drian the noodles and place them on a clean tea towel or on several layers of kitchen paper, to get rid of the excess moisture.
Tip the noodles into a large bowl and mix together with the cucumber, coriander, nori squares, sesame seeds and pickled ginger.
Toss the salad with the dressing and divide between two plates. Slice the tuna, arrange it next to the salad and serve.