Cut the Napa cabbage lengthwise into quarters, then chop it into bite-sized pieces. In a large bowl, sprinkle salt over the cabbage, mixing well to coat evenly. Let it sit for about 2 hours to soften and draw out moisture.

After 2 hours, rinse the cabbage under cold water to remove excess salt and drain thoroughly.

In a small bowl, combine the minced garlic, ginger, and red pepper flakes to create a spicy paste. Adjust the amount of red pepper flakes according to your preference.

In a larger bowl, combine the drained cabbage, julienned carrots, radish, and chopped green onions. Add the salt-and-pepper paste and mix until all the vegetables are well coated.

Transfer the vegetable mixture into a clean, airtight jar or container. Press down to ensure the mixture is tightly packed, leaving about an inch of space at the top for the mixture to expand as it ferments.

Leave the jar at room temperature for 2-3 days. Taste along the way to check for preferred fermentation level. After that, move the jar to the refrigerator, where it should stay good for about a month.