Salt the cabbage: Cut a slit into the base of the Napa cabbage, about 3 inches deep, and tear in half. Repeat with each half, cutting a small slit and tearing again to create quarters. Into a large bowl, mix 5 cups cold water + ¼ cup coarse sea salt. Dunk and swish each cabbage quarter in the brine to coat the leaves. Sprinkle pinches of the second ¼ cup salt between every 2–3 leaves, focusing on the thick white stems. Place salted cabbage in a separate large bowl, pour leftover salt water over the top, press down, and cover. Let sit 4 hours in summer (6–8 hours in winter) away from direct sunlight. Flip and press halfway through.

Rinse & drain: Rinse cabbage 3–4 times in clean water until it tastes pleasantly salty. Using a knife, cut out just the tough, thick base of the core (about 1–2 inches), but leave enough so the leaves stay connected at the bottom. Place the cabbage in a strainer or colander and let it drain well while you prepare the brine.

Make the flour porridge: In a small pot, whisk 1 cup water + 1 tbsp flour. Heat over medium, stirring occasionally to prevent sticking. The mixture will start to thicken as the flour cooks. Once it begins bubbling in the center (~8–9 minutes), reduce the heat to low and cook for another 2 minutes. Remove from heat and let it cool completely before using.

Blend the marinade: Blend ½ lb radish, 1 red apple, ½ sweet onion, 8 garlic cloves, 1 tsp ginger until smooth. Into a medium bowl, strain the blended marinade through a cotton cloth/cheesecloth (or fine mesh) to extract a clear, light juice. Into the medium bowl, whisk in cooled porridge, 2 cups water, 1 tbsp fish sauce, and 1 tbsp salt. The liquid should be slightly beige. Taste the liquid — it should be slightly savory, slightly sweet, and refreshing. Adjust with a little extra water or salt if needed.

Assemble the kimchi: In your container, layer 2 cabbage quarters, sprinkle half the green onions, add the last 2 quarters, sprinkle the rest of the green onions, and pour the marinade over, using a strainer to catch any blended solids. Press cabbage down so it’s submerged.

Ferment: Leave at room temperature for 1–2 days, depending on how warm your kitchen is. You’ll know it’s ready to move to the fridge when the cabbage looks more translucent, the brine smells lightly tangy, and you see tiny bubbles near the top. Chill and store in the fridge.