Boil water, cook buckwheat noodles for 4-5 minutes or according to package instructions. While cooking, air out the noodles to keep them from sticking.
Drain and rinse with cold water. Leave it on a strainer to drain, but add ice cubes to keep it cold.
In a bowl, combine tsuyu, water, fish sauce, and sugar.
Grate or blend radish, squeeze out excess water, then form into balls.
Plate the noodles by portioning them, then garnish with seaweeds and sesame seeds.
Assemble the sauce by portioning the tsuyu sauce, then garnish with grated radish, scallions, and sesame seeds.
Serve noodles and sauce separately.
To eat, customize the sauce to your liking, dip each bite of noodles, and enjoy!