In a medium saucepan, add 1½ cups water and heat until nearly boiling.
Add 1 cup katsuobushi (dried bonito flakes) and let it simmer for 15 seconds, and then turn off the heat. Set aside and let the katsuobushi steep for 15 minutes.
Meanwhile, peel the skin of 7 oz daikon radish, grate it (I use a ceramic grater), and set aside.
Cut 1 green onion/scallion into fine rounds.
After 15 minutes of steeping, strain the stock into a measuring cup. You should have 1¼ cups of katsuo dashi. If not, add water.
Add 1 Tbsp mirin, 1 Tbsp sugar, and 3 Tbsp soy sauce. Bring it to boil on medium heat and turn off the heat.
Mix well and taste it. If necessary, season with ¼ tsp Diamond Crystal kosher salt.
Place the pot in iced water to cool the sauce down before transferring to the refrigerator (if you are planning to serve later on).
To Cook the Soba
Bring a large pot of water to a boil. For cooking soba noodles, we do not salt the water. Also, remove the paper bands on 6 oz dried soba noodles (buckwheat noodles) ahead of time. To cook all the soba equally, it’s best to prepare and have it ready to go.
Once the water is boiling, add the soba and quickly stir so the noodles don’t stick to each other. Cook the soba noodles according to the package instructions. Keep the water boiling on medium to medium-high heat.
Drain the soba noodles (you can reserve the soba cooking water called sobayu for drinking). Rinse the soba noodles well under cold water to remove the starch.
Serve the noodles in individual dishes, and divide the sauce.
Now, prepare the toppings. Gently squeeze the grated daikon and place on top of the noodles. (Don’t squeeze too hard and save some moisture so it’s more refreshing, but too much liquid will dilute the sauce.) Top with ½ cup katsuobushi (dried bonito flakes), 3 Tbsp tenkasu/agedama (tempura scraps), the sliced green onion, and 1 Tbsp shredded nori seaweed (kizami nori). Serve immediately.