Wipe the kombu with a dumb paper towel to remove possible sand on the surface.
Put the kombu and 1800ml of water in a saucepan, then start heating over medium-low heat. ※Soak the Kombu in the water overnight if you want a richer kombu flavour.
Just before the water comes to a simmer, take out the Kombu.
Bring the water to a boil and put a mesh strainer in the saucepan. Then, add the Katsuobushi and turn off the heat. Leave the Katsuobushi immersed in water for 5 minutes.
Lift up the mesh strainer and let the dash drain itself, but do not press the Katsuobushi as it causes bitterness. You should end up having 1200 to 1500 ml dashi stock.
Add the white soy sauce. Check the taste and add more soy sauce if necessary.
Add the Mirin
Cut the beef rib slices into 10cm long pieces.
Add them in the dashi stock and heat over high heat.
Once it comes to a boil, turn down the heat and skim off the scum well. Simmer over medium low heat for 5 minutes.
Start heating the water for boiling the Udon noodles.
In the meantime, bias slice 3 scallions and finely chop the other 3 scallions.
Add the bias sliced scallions into the beef soup.
Boil a portion (220g) of Udon noodles for a minute.
Add the noodles into a serving bowl and pour over the soup.
Sprinkle some of the finely chopped scallions.
Finally, sprinkle a touch of Shichimi-Togarashi (Japanese seven-spice powder).