Cook soba according to directions, Drain, then rinse in cold water. Combine soy and mirin, then toss through noodles. Brush tuna with oil, then season, Heat a large, non-stick fry pan over high heat. When hot, cook tuna for 20-30 seconds each side until lightly coloured but still completely rare inside. Remove from pan and cut into 55mm-thick slices. Divide the noodles among plates, then top with spinach, broccolini and tuna.