Rinse the cooked rice briefly in a strainer.

Extract the contents of the mentaiko and reserve half for garnish. Trim the stem of the shiso leaf.

Peel the nagaimo and place it in a plastic bag. Mash it with a rolling pin until smooth (tororo).

In a microwave-safe bowl, combine the rice and seasoning ingredients (shiro dashi and water). Cover loosely with plastic wrap and microwave at 600W for 2 minutes.

Remove the wrap, add the beaten egg, and mix lightly. Cover loosely again and microwave at 600W for another 2 minutes. Stir in half of the mentaiko.