Place cabbage in a very large bowl. Mix together sea salt and water until the salt has dissolved. Pour over the cabbage and mix thoroughly with your hands. Let it sit for 1.5-2 hours.

Blend the onion and garlic with ¼ cup of water to create a puree. Pour this into a medium bowl and mix it with red pepper powder and green onions until evenly combined.

Once the cabbage has wilted, rinse it to remove most of the salt water. Return the cabbage to the bowl and toss it with the seasoning mixture until well-coated.

Place the seasoned kimchi into a large mason jar, pressing down to compress it. Ensure the jar is filled completely, and use another jar if needed.

Tightly close the lid and leave the jar out at room temperature overnight to start the fermentation process.

Taste the kimchi the next day. For a more sour flavor, leave it out for another day. Otherwise, refrigerate it.