Step 1: Prepare the Napa Cabbage. Quarter the napa cabbage lengthwise, then chop each quarter into 2-inch pieces. Rinse thoroughly under cold water.
Step 2: Salt the Cabbage. In a large, clean basin or bowl, sprinkle coarse sea salt over the cabbage pieces, tossing to ensure even coating. Let it sit for 1.5 to 2 hours, tossing every 30 minutes. The cabbage will wilt and release a lot of water.
Step 3: Rinse and Drain. After salting, thoroughly rinse the cabbage under cold running water 2-3 times to remove excess salt. Squeeze out as much water as possible without bruising the cabbage. Set aside to drain in a colander for at least 30 minutes.
Step 4: Prepare the Flavor Paste. In a food processor or blender, combine garlic, ginger, onion, and apples or Asian pear with a splash of water until a smooth paste forms. In a large mixing bowl, combine this paste with gochugaru, fish sauce (or soy sauce), sugar, and optional glutinous rice flour paste (if using, cook 1 tbsp glutinous rice flour with 1/2 cup water until thick, then cool). Mix well.
Step 5: Add Vegetables. Add sliced radish, carrots, and green onions to the flavor paste. Mix everything thoroughly.
Step 6: Combine Cabbage and Paste. Put on a pair of food-safe gloves! Add the drained cabbage to the bowl with the paste and vegetables. Using your hands, thoroughly massage the paste into every single piece of cabbage, ensuring even distribution.
Step 7: Pack and Ferment. Tightly pack the kimchi into clean glass jars or a fermentation crock, leaving about an inch of headspace from the top. Press down firmly to remove any air pockets and ensure the cabbage is submerged in its brine. Close the jar loosely (or use an airlock for crocks).
Step 8: Fermentation. Let the kimchi ferment at room temperature (around 65-70°F or 18-21°C) for 1-5 days. The exact time depends on your desired level of sourness and the ambient temperature. Taste daily after the first 24 hours. Once it reaches your preferred taste, transfer it to the refrigerator.