Start by removing any wilted outer leaves from the Napa cabbage and carefully quarter it. Rinse under cold water and drain.

Generously sprinkle kosher salt between each layer of cabbage quarters and stack in a mixing bowl, weighing down for 1½ to 2 hours.

Prepare the kimchi marinade by cooking rice flour with water until thickened, then blend with apple, onion, garlic, ginger, and fish sauce.

Rinse the cabbage to remove excess salt and drain thoroughly, gently squeezing each quarter.

Rub the kimchi marinade onto each leaf of the cabbage, ensuring even coating, then pack tightly into jars.

Let the kimchi ferment at room temperature for 1–2 days before transferring to the refrigerator.