Cut off both ends of the cucumber. Slice it in half lengthwise, place it cut-side down, and slice thinly at an angle. In a bowl, rub the cucumber with salt and let it sit for 5 minutes. Squeeze out excess water. Cut the ginger into thin strips.

Separate the mackerel from its juice. In a bowl, roughly flake the mackerel. Add the cucumber, ginger, sesame seeds, and the seasoning ingredients (mackerel juice, soy sauce, sesame oil). Mix well.