Slice the shiso leaves thinly. Place the nagaimo in a plastic bag and lightly crush it with a rolling pin, leaving some chunks.

Cut the tarako lengthwise and scoop out the contents with a spoon.

Boil water in a pot and cook the somen noodles according to package instructions. Rinse under cold water and drain well.

Mix the mentsuyu, water, and sesame oil in a bowl.