Chop the onion into coarse pieces. Place it in a microwave-safe container, cover loosely with plastic wrap, and heat at 600W for about 1 minute. Drain excess water and let it cool. Mince the red pickled ginger.

Place the fish cake in a plastic bag and mash until smooth. Add the remaining filling ingredients and knead thoroughly. Divide into 8 portions and shape into oval patties.