Soak the kombu for about 1 hr before you start cooking or leave overnight to have it ready the day you cook. If you’re short on time, just soak it as you start making the soup.
Boil 2 eggs until hard. Once cooked, set them aside.
In a pot, add the kombu and the kombu water and bring to a boil on medium heat. When the water starts to boil, remove the kombu pieces and set them aside for later use or discard.
Now add in the bonito flakes and simmer for approximately 30 seconds.
Turn the heat off and let the bonito flakes simmer in the dashi for 10 minutes.
After the 10 minutes are up, drain the stock and remove all pieces. You should now have a light yellow clear broth.
Add in the soy sauce, mirin, sake, and salt, and stir. Bring it to a simmer and then put it aside.
Slice the green onion or scallions into small pieces.
Peel the eggs and cut them into halves.
Bring water to a boil and cook the soba noodles per packaging instructions or usually 4-5 minutes.
Drain the soba noodles and run them under cold water for a few seconds to remove the remaining starch.
Place noodles in the bowls, add the dashi soup and garnish with spring onions and two egg halves. Now you’re ready to enjoy the hot soup!