Grate the daikon, and thoroughly drain the moisture. Mix the included sauce (and the mustard to taste) into the natto until evenly incorporated.
Boil the soba and put it in a serving bowl.
Combine the mentsuyu and the dashi and bring to a boil. Pour into to the bowl with the soba.
Add the natto, and top with the grated daikon. Sprinkle in the green onions and it's complete.
Natto can be freeze-stored (Recipe ID: 520102).
Story Behind this Recipe
A coworker from Hiroshima at my old work place taught me about natto soba. I wanted to eat a version with vegetables, and without the natto sinking to the bottom of the bowl! So, I came up with my own variation. I didn't really like soba before, but I've been buying it more since learning this recipe. Of course, this is what I have for my New Year's Eve soba!
To dilute the noodle dipping sauce, be sure to use dashi, not water. The taste of the soba sauce will be completely different. The reason for using half the regular amount of sauce is because the natto will sink as you eat it. Another reason is to keep it from becoming too salty; if you prefer a lot of sauce, then double the amount. This is also delicious with chilled noodles and sauce.