Add the napa cabbage to a large bowl and sprinkle salt and sugar over the top. Add 4 to 6 cups warm water, or just enough to submerge the cabbage when you press down on the cabbage. Set aside for 30 minutes.
Reserve 1 cup of the salted water. Drain the remaining water and rinse the cabbage to remove any excess salt and sugar.
In another large bowl, stir together the fish sauce, sesame oil, rice vinegar, Gochugaru, garlic, and ginger.
Add the cabbage, carrots, and onions, and fold to thoroughly combine.
Pack cabbage mixture tightly into a 1-quart wide mouth jar with plastic lids, pressing down on the kimchi until the brine rises to cover the vegetables, leaving at least 1" of space at the top. If needed, add a little bit of the reserved salted water to cover the cabbage.
For a milder flavor, place the kimchi in the refrigerator to ferment right after packing. For a stronger flavor, leave it on the counter in a cool, dark place with the lid slightly ajar for up to two days. You may see bubbles inside the jar and brine may seep out of the lid. Then store in the refrigerator to further ferment.
