Remove outer leaves of cabbage and set aside for later. Shred the cabbage using a mandolin slicer or kitchen knife.
If using, peel and core the apple. Cut into thin strips or grate with a box grater. Add to the cabbage.
Weigh the cabbage (and apple). Add 2% of that in sea salt. Mix well.
Let sit at room temperature for about one hour or until the cabbage looks moist and glossy.
Using a tamper, press the cabbage mixture into mason jars or sauerkraut crock.
When the container is almost full, place one or two cabbage leaves on top. Place fermentation weights on top to keep everything submerged.
Place a fermentation lid on. Let the vegetables sit in a dark place at room temperature for 3-6 days without opening the jars. Enjoy or let sit for a total of 30 days for maximized probiotics.