Cut tuna into logs. Roll the outside edges of each piece in the shichimi togarashi. Place oil in a pan on a medium-high heat. Sear on all sides (about 30 seconds each side) until it has a cooked edge about 5mm thick, keeping the centre raw. Allow to cool in refrigerator while you prepare the other ingredients.
To make the dressing, mix the sesame paste and soy sauce in a bowl. Add the vinegar and sugar, stirring to dissolve. Whisk through garlic, ginger and sesame oil and allow to stand for 30 minutes to allow the flavours to develop.
Finely shred the cabbage and slice the spring onions. Cut the carrots into matchsticks. Pick and wash the coriander leaves. Combine the salad ingredients with toasted nuts and seeds.
Slice the tuna into 5-8mm pieces. Dress the salad, then place on four plates, top with sliced tuna and serve.