Cut open the mentaiko and extract the contents.

Bring the chicken breast to room temperature, slice it horizontally to open, and flatten lightly with the back of a knife. Season both sides with salt and pepper, then layer shiso leaves, melting cheese, and mentaiko inside the chicken.

In a bowl, whisk the egg, add flour and water, and mix to create the batter.

Dip the stuffed chicken into the batter and coat with breadcrumbs, pressing lightly to ensure they stick.

Heat vegetable oil in a pot to 140–150°C (about 3cm deep). Fry the chicken on medium-low heat for 7–8 minutes until cooked through, then let rest for 5 minutes to drain excess oil.