Wash the cabbage leaves, and discard any discolored parts. Dry by spinning in a salad spinner or in several layers of paper towels. Chop up roughly.
Mix together all the dry ingredients and orange zest in a bowl. If you’re using a stock cube, crumble it up finely.
Put the cabbage in the plastic bag, and the dry ingredients + orange zest. Close the bag with plenty of air in it (so it’s like a balloon) and shake all around until the ingredients are evenly distributed.
Open the bag, expel as much air as you can, and close again. Massage and toss the bag around, bruising the cabbage. Quite a lot of moisture will come out of the cabbage and melt the salt etc. Try not to burst the bag with over-enthusiastic kneading.
You can eat this right away, or leave in the refrigerator in the bag for about an hour (you can do it over night too).
To serve, drain off the excess moisture (which is delicious, but rather salty…though some people like to drink it off!). Drizzle with the optional lemon juice or sesame oil. Serve with any Japanese style meal. Makes a great bento box item.