Gather all the ingredients (the garnishes and optional toppings are not shown).
Combine ¼ cup mentsuyu (concentrated noodle soup base) and ½–1 cup iced water in a measuring cup and mix well.
Cut 1 green onion/scallion into thin slices and grate the ginger.
Thinly slice shiso leaves and myoga ginger, if using, and place them in separate small bowls.
Grate a small amount of daikon radish, gently squeeze some of the liquid out, then place in a small bowl.
Pour some toasted white sesame seeds into a small bowl.
To make the simmered shiitake mushrooms, remove the stems from shiitake mushrooms and cut the caps into thin slices.
In a saucepan, combine the mushroom slices, soy sauce, mirin, sugar, and water (or dashi). Simmer until there is almost no liquid left.
Bring a large pot of water to a boil.
Remove all the wrappings from the dried somen noodles.
When the water is boiling, add the somen all at once, spreading the noodles around the pot.
Cook in the boiling water for 1 to 1½ minutes or according to the package instructions.
Drain the noodles in a colander and rinse them under cold running water.
Transfer the somen to a large bowl of iced water with several ice cubes.
Serve the noodles either chilled in iced water or on a tray with the tsuyu dipping sauce in small individual bowls.
