Add sake in to a small saucepan and let the alcohol evaporate over medium heat.
Add mirin and soy sauce to the saucepan.
Add the kombu and katsuobushi (bonito flakes).
Bring it to boil and lower the heat. Let it simmer for 5 minutes on low heat. Turn off the heat and set aside to cool. Strain the mixture through a fine mesh sieve. The concentrated sauce is ready to use or keep for a month in the refrigerator.
Peel daikon and ginger and grate them. Place them on a small plate.
Cut the green onion thinly, and put it on the plate along with grated daikon and ginger. You can also garnish with shredded nori seaweed and wasabi.
Cook the soba noodles according to the package instructions.
Drain the water and rinse the noodles under cold water to remove the starch.
Combine concentrated sauce and iced water (1:2) to make dipping sauce.
Placed the soba noodles on a plate and pour the dipping sauce in a smaller bowl/cup. Serve with the plate of garnish.
“Tsuyu”, a noodle soup base, makes 1 cup concentrated sauce. For this recipe, you will need about half portion. Keep the half for later use without diluting water. Concentrated sauce can be kept in the refrigerator up to a month.
You can reserve 2 cups of the soba cooking water. Japanese call this liquid “Sobayu” and it is served at the end of meal. “Sobayu” is used to dilute the dipping sauce and enjoy it as hot soup after eating soba.