Peel the skin off the yamaimo and grate. Chop the scallions and set aside.
Follow the package instructions of the udon to cook and prepare the soup. Pour the udon and soup in a bowl.
Add the grated yamaimo (Step 1) on top of the udon. Add the egg yolk to the center and top with chopped scallions, nori seaweed, and wasabi. Mix together and enjoy!*You can also add soy sauce and mix into the grated yamaimo for more flavor!