To make mentsuyu: to remove any excess saltiness from the kombu, either lightly wash the kombu seaweed uner running water then pat dry or wipe with a damp cloth. Put 3 1/2 cups water in a saucepan, add the kombu, and leave for 30 minutes to 1 hour, depending on the thickness of the kombu
Add the soy sauce, mirin, and sugar to the water and kombu, and place over medium heat
Just before it comes to a boil add the katsuobushi and boil for 2-3 minutes, after which remove from the heat and let it stand until all the flakes have sunk to the bottom of the saucepan. Then strain into a bottle or a jar, allow to cool and then refrigerate
Peel and finely grate the daikon so it looks like mashed potatoes
Bring a large saucepan of water to a boil and add the soba noodles, being carefully they do not stick in the water. When it comes to a boil, add a cup of cold water-this will help ensure the soba noodles are cooked perfectly
Bring back to a boil and cook according to the package instructions
When cooked, strain and rinse under running cold water, to stop the noodles from becoming too soft. Drain thoroughly
Divide the noodles into individual serving bowls, place some grated daikon on top, and add the mentsuyu by carefully pouring it in at the edge of each bowl
Garnish with nori seaweed, sliced scallions, and wasabi