Place a plastic wrap on a working surface and put a sheet of nori seaweed on top (shiny side facing down), with a corner pointing up.
Evenly spread the steamed rice in a thin layer and form into a square shape in the center of nori sheet. Sprinkle a little bit of salt. For Japanese rice ball (onigiri) that are enjoyed at room temperature, we add salt to rice keeps the food fresh longer.
Place the shredded cabbage evenly on top of rice and then the chicken katsu on top.
Spread the mustard and tonkatsu sauce.
Flip over chicken katsu so the sauce is also on the cabbage.
Place steamed rice on top. Try to keep the square shape as you spread the rice evenly in a thin layer. Bring left and right corners of nori sheet towards the center. Fold gently but tightly to wrap around the layers at the center.
Then bring bottom and top corners towards the center. Continue to fold gently but tightly around the layers. Make sure the rice is tucked in nicely.
If you like the onigirazu to showcase the filling after cutting in half, mark with a piece of shredded cabbage perpendicular to the filling. Wrap with plastic wrap tightly and set aside for 5 minutes.
Cut the onigirazu with a sharp knife (following the direction of the “cabbage” mark). Run your knife in cold water before cutting so that the cross-section will be clean.
If you plan to make it ahead of time, wrap the onigirazu with a thick kitchen towel and keep it in the refrigerator overnight. The towel will prevent the rice from getting hard and dry from cold air (but it's impossible to keep it moist/fluffy). I strongly recommend making onigirazu on the same day you enjoy it.