Infuse kombu in 200ml water for 30 minutes, then add soy sauce and mirin, boil, add bonito flakes, and cool.
Strain the dipping sauce once cooled and chill in the fridge.
Boil soba noodles as per package instructions. Reserve cooking water, rinse noodles under cold water, and drain.
Serve with spring onions and wasabi in the dipping sauce. Dip noodles and enjoy slurping! Use reserved cooking water in the dipping sauce after finishing noodles.
Notes
This dish is best enjoyed cold, and the reserved water can be consumed as a nourishing soup.