Combine sake and mirin in a microwave-safe container. Heat uncovered at 600W for 30 seconds to evaporate alcohol. Let cool and mix in soy sauce to create the marinade.

Separate the squid's body from its innards by inserting a finger to detach the connecting muscle. Pull out the tentacles and innards gently and remove the cartilage.

Prepare the tentacles by cutting below the eyes, removing the beak, and cleaning the suction cups. Trim the long tentacles to match the shorter ones and cut into 3cm pieces.

Detach the fins from the body, open the body by slicing lengthwise, and clean the inside with a paper towel. Cut the body into 1cm strips and the fins into 1cm wide pieces.