1Cut the napa cabbage lengthwise into quarters and remove the cores. Chop each quarter into bite-sized pieces.
Place the cabbage in a large bowl and sprinkle with the sea salt. Toss to combine, then let it sit for 1-2 hours, tossing occasionally, until the cabbage has softened.
Rinse the cabbage under cold water to remove excess salt and drain well.
In a separate bowl, combine the sugar, fish sauce, ginger, garlic, and Korean red pepper flakes to form a paste.
Add the drained cabbage and green onions to the paste and mix thoroughly, ensuring the cabbage is well coated.
Pack the kimchi into a clean jar, pressing down to remove air pockets, and leave some space at the top for expansion.
Seal the jar and let it ferment at room temperature for 1-5 days, depending on your taste preference. Check daily and press down the kimchi to keep it submerged in its juices.
Once fermented to your liking, store the kimchi in the refrigerator to slow down the fermentation process.