Cut the napa cabbage lengthwise into quarters and remove the core.
Cut each quarter crosswise into 2-inch wide strips.
Place the cabbage in a large bowl and sprinkle with sea salt, tossing to coat evenly.
Add enough water to cover the cabbage and let it sit for 1-2 hours, turning occasionally to ensure even salting.
Rinse the cabbage thoroughly under cold water to remove the excess salt, then drain well.
In a small bowl, combine the Korean red chili pepper flakes (gochugaru), sugar, fish sauce, soy sauce, minced garlic, grated ginger, and grated onion. Mix well to form a paste.
Add the chopped green onions, julienned carrot, and julienned daikon radish to the kimchi paste. Mix until all the vegetables are well-coated with the paste.
In a large bowl, combine the drained cabbage with the kimchi paste mixture.
Using your hands (wearing gloves to protect from the chili paste), gently work the paste into the cabbage, ensuring all pieces are thoroughly coated.
Pack the kimchi into a clean, airtight jar or container, pressing down to remove any air bubbles. Leave about 1 inch of headspace at the top.
Seal the jar tightly.
Let the kimchi ferment at room temperature for 1-2 days, depending on the temperature and your taste preference.
Once it reaches the desired level of fermentation, transfer the kimchi to the refrigerator to slow down the fermentation process.
Kimchi can be eaten fresh or aged, and it will continue to develop its flavor over time.
Enjoy your meal!