Cut the Napa cabbage lengthwise into quarters and remove the cores. Chop the quarters into 2-inch wide pieces.
In a large mixing bowl, dissolve sea salt in water. Add the chopped cabbage and ensure it's fully submerged in the saltwater. Place a heavy object on top to keep the cabbage down. Let it sit for 1-2 hours.
Rinse the cabbage under cold water 3 times to remove excess salt. Drain well and set aside.
In a separate bowl, combine minced garlic, grated ginger, fish sauce, and sugar substitute to create the kimchi paste.
Add the sliced green onions and julienned daikon radish to the paste, mixing until they are well coated.
Sprinkle the Korean red pepper flakes over the vegetables and mix thoroughly, ensuring even distribution of the spice.
Add the drained cabbage to the spicy mixture and, wearing gloves, massage the paste into the cabbage until it is completely covered.
Pack the kimchi tightly into a clean, airtight fermenting jar, pressing down to eliminate air pockets. Leave at least 1 inch of space at the top.
Seal the jar and let it sit at room temperature for 2-5 days, checking daily to press down any cabbage that may float to the top. The kimchi is ready when it's fermented to your liking.
Once fermented, store in the refrigerator to halt the fermentation process. The flavor will continue to develop as it ages.