Cut open the mentaiko and extract the contents.

Peel and dice the potato into 1cm cubes, rinse in water, and drain. Microwave in a heatproof container covered with plastic wrap at 600W for 3 minutes. Mash while hot, then mix with milk and mentaiko until smooth.

Layer shiso leaves in the width of the omelette pan, place the mashed potato mixture on top, and roll.

In a bowl, mix the egg mixture ingredients (eggs, sugar, salt, dashi powder, and water) by cutting through with chopsticks.

Heat salad oil in the omelette pan over medium heat. Pour 1/3 of the egg mixture, place the rolled potato mixture at the back, and roll. Push to the back, pour half of the remaining egg mixture, and roll forward. Repeat with the remaining egg mixture.