Prepare the Cabbage: Remove outer leaves, cut into quarters, remove the core, and slice into thin shreds. Place shredded cabbage in a large bowl and sprinkle with sea salt. Massage salt into the cabbage and squeeze out excess juices (5-10 minutes) to create brine. Mix in any desired optional ingredients.
Transfer to Container: Place salted cabbage in a clean glass container. Leave space at the top for bubbling. Cover with a clean cloth to allow gases to escape.
Fermentation: Let the container sit on a countertop at 18-22°C (64-72°F) for 1-2 weeks, out of direct sunlight. If the temperature is high, ferment for 1-2 days at room temperature, then refrigerate.
Taste and Store: After one week, taste the sauerkraut. Once it reaches the desired sourness, seal it with an airtight lid and enjoy!