Usukuchi soy sauce (or regular soy sauce, if unavailable)
2 to 3 cubes
Dissolve the salt and dashi stock granules in hot water. Add soy sauce and cold water to prepare the soup for the noodles. I recommend preparing this beforehand and keeping it chilled while boiling the noodles.
Use a mandolin or similar to thinly slice the sudachi citrus. (Be careful not to cut your hands!) Grate the daikon radish (optional).
Boil the udon noodles as instructed on the packaging.
Once boiled, rinse the starch off the noodles under cool running water and chill.
Portion the noodles out into bowls, pour in the cold soup, and top with the sliced sudachi citrus and ice cubes. Add daikon radish as desired and enjoy.
You can upgrade the flavor by preparing your own dashi stock from scratch with bonito flakes. Speed up the work by making some the night before and keeping it chilled in the fridge.
Story Behind this Recipe
I had a similar dish at the soba noodle restaurant in my neighborhood and thought the same idea might work with udon.
This is just as delicious with somen, hiyamugi, or soba noodles. The ingredients are simple, so I recommend using a good dashi stock. It's also tasty to use diluted mentsuyu (noodle dipping sauce) with additional salt as preferred.