Cut the mentaiko and extract the roe.

Thinly slice the shiso leaves.

Peel the nagaimo and place it in a plastic bag. Pound it with a rolling pin until slightly chunky.

Add mentaiko and mentsuyu to the bag with nagaimo and mix until uniform.

Place frozen udon in a microwave-safe container, cover loosely with plastic wrap, and heat at 600W for the time indicated on the package. Rinse with water and drain.