Mince the onion. Cut the potatoes into bite-sized pieces and soak them in water to remove excess starch. Remove the contents of the mentaiko by slicing it open.

Place the onion and potatoes in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 7 minutes until softened.

Mash the hot potatoes, then mix in mayonnaise and mentaiko. Divide into 4 portions, shape them, and refrigerate for 30 minutes.

Coat the croquettes in flour, then beaten egg, and finally breadcrumbs.