Quarter the napa cabbage lengthwise, then chop into 2-inch pieces.

In a very large bowl, dissolve 1/4 cup sea salt in 8 cups of cold water. Add the chopped cabbage, ensuring it is fully submerged. Let the cabbage brine for 2-3 hours at room temperature, tossing occasionally, until the leaves have softened significantly.

Drain the cabbage in a colander, then rinse thoroughly 3-4 times under cold running water to remove excess salt. Squeeze gently to remove as much water as possible from the cabbage without tearing the leaves.

In a separate large mixing bowl, combine the gochugaru, minced garlic, grated ginger, sugar, and fish sauce (or soy sauce) to form a thick, spicy paste.

Add the well-drained cabbage, chopped green onions, and julienned daikon radish (if using) to the paste. Wearing food-safe gloves, thoroughly mix the paste into the cabbage and other vegetables, ensuring every piece is coated evenly.

Pack the kimchi tightly into a clean, sterilized glass jar, pressing down firmly to remove air pockets and ensure the cabbage is submerged in its brine. Leave about an inch of headspace at the top.

Cover the jar loosely with a lid or use an airlock. Let the kimchi ferment at room temperature (ideally 65-70°F or 18-21°C) for 1-5 days, tasting daily until it reaches your desired level of sourness and fizz. The longer it ferments, the tangier it will become.

Once fermented to your liking, secure the lid tightly and transfer the kimchi to the refrigerator. It will continue to ferment slowly and can be stored in the fridge for several weeks to months. Enjoy as a side dish or ingredient in various Korean recipes.