Marinate tuna in soy,mirin, ginger and chilli for 20- 25 minutes to allow flavours to infuse. Remove the tuna and pat dry, keeping the marinade for dressing. Sear tuna quickly [approx 30-40 seconds on each side in a hot pan]. Remove from pan and place on a chopping board and rest for a further minute. While tuna is resting, deglaze the pan with water. Add this to the marinade and toss with the soba noodles and spring onions [this noodle salad is best served at room temperature] Slice tuna, serve on soba Salad and garnish with toasted seasame seeds.