Start by peeling away any damaged outer leaves from the Napa cabbage and cut it into quarters. Rinse the cabbage thoroughly under cold water to remove any dirt.

Generously sprinkle kosher salt between the cabbage leaves, then place in a large bowl with weight on top to keep submerged. Let sit for 1.5 to 2 hours, turning every 30 minutes.

In a small saucepan, cook the rice flour with water over medium heat, stirring until it thickens to a paste. Allow to cool.

In a blender, combine the apple, onion, garlic, ginger, and fish sauce, then blend until smooth. Mix with rice flour paste and gochugaru until well combined.

After salting, rinse cabbage under cold water to remove excess salt and gently shake off the water.

Put on gloves to protect your hands from the spicy gochugaru. Massage the marinade between cabbage leaves, distributing julienned radish, carrot, and chives evenly.

Pack the mixed cabbage tightly into sterilized jars, leaving space at the top for fermentation gases. Press down to eliminate air pockets.

Cover jars with a lid and let ferment at room temperature for 1 to 3 days, tasting daily to reach preferred sourness.