Prearing the mentsuyu (dipping sauce)
Prepare mentsuyu according to this recipe. Allow to cool in the refrigerator for 1 hr.
To cook
Cook the soba only right before serving. Drain the soba in a colander and rinse the noodles under cold, running water until the noodles are cooled. Transfer to a bowl filled with cold water.
To serve, divide the soba among 4 plates and sprinkle nori on top. Pour the dipping sauce into 4 small cups with the 1 tbsp spring onions and a dab of wasabi paste (optional) on the side of each plate. Reserve the remaining mentsuyu for top-ups.
To eat, add the spring onions to the mentsuyu. Dip the soba into the mentsuyu and make sure to slurp when enjoying the noodles!