Place nagaimo in a plastic bag and crush it with a rolling pin. Cut the roots off kaiware sprouts and halve them.

Cut open the mentaiko and extract the roe, reserving a small amount for garnish.

Mix hot water and dashi powder in a bowl to prepare dashi. In another bowl, combine crushed nagaimo, dashi, mentsuyu, and mentaiko to make tororo sauce.

Boil somen noodles according to package instructions. Rinse under cold water to remove starch and drain well.