Wash the napa cabbage and slice it into pieces about 2 inches in length. Place the cabbage in a large container, add the salt, and cover with water. Soak for 1 1/2 hours, rotating and mixing every 30 minutes. Rinse twice and set aside to drain.

In a pan, whisk together 3 cups of water and the sweet rice flour until well combined. Bring to a boil, then reduce heat and simmer until it reaches a paste-like consistency.

Add the sugar to the rice flour paste and mix well for 2 to 3 minutes. Remove from heat and let it cool completely.

In a blender, combine the minced onions, crushed garlic, minced ginger, and fish sauce. Blend until you achieve a creamy consistency.

In a large mixing bowl, combine the cooled rice porridge, the blended onion sauce, and the Korean hot pepper flakes (gochugaru).

Add the drained cabbage, carrots, green onions, and Korean radish to the spice paste. Mix thoroughly by hand until all vegetables are evenly coated.

Transfer the mixed kimchi into airtight jars or containers. It can be eaten fresh immediately, or left on the counter at room temperature for 2 days to undergo the fermentation process.

After 2 days, transfer the fermented kimchi to the refrigerator for storage.