Mix together sea salt and water in a large bowl and stir until salt has dissolved.
Add cabbage and let it sit in this salted water for about 1-2 hours or until it's significantly wilted.
In a blender, add onion, garlic, ginger, fish sauce, sugar, red pepper powder, water and pulse until you have a smooth paste. Keep aside.
Drain and rinse cabbage in cold water to remove most of the salt water.
Add it back in the same bowl (just rinse it).
Add the red pepper paste, green onions and mix until cabbage is coated well.
Transfer this seasoned kimchi to a large mason jar and push it down and keep stuffing it until it's completely full. Use another jar, if necessary.
Tighten the lid and let it sit at room temperature.
Taste it the next day with a clean spoon or fork and if you want it more sour and ripe, leave it out for another 3-4 days. Check daily until you are satisfied with the flavor. You may see bubbles inside the jar and some brine may seep out of the top of the lid. That's normal and means the fermentation is working. For me, 2 days is best.
When the kimchi tastes sour and ripe enough to your liking, transfer the jar from room temperature to to the fridge. Enjoy!