Cut the chicken breast into bite-sized pieces.

Slice open the mentaiko and extract the contents. Mix half of the mentaiko with the beaten egg and flour in a bowl to create the batter.

Mash the boiled egg in another bowl using a fork. Add the remaining mentaiko, chopped green onion, mayonnaise, and milk to create the tartar sauce.

Heat vegetable oil in a frying pan over medium heat. Coat the chicken pieces in the batter and pan-fry until both sides are golden brown and cooked through.