Reminder: When fermenting, all utensils, dishes, hands, etc. should be VERY clean and sterile.
To begin, remove the whole outer leaves of the cabbage. Set them aside for now.
Thinly slice the rest of the head of cabbage and discard the stem.
Place the sliced cabbage in a large bowl and sprinkle in the sea salt. Using your hands, massage the salt into the leaves until fully incorporated. The cabbage will begin to soften and release its juices.
Add enough dechlorinated water to just barely cover the cabbage.
Place another bowl (the same size) on top of the cabbage. Fill a large jar of water and set it in the tope bowl to weigh it down and keep the cabbage submerged. Allow this to soak 4 - 12 hours.
Drain the brine from the cabbage with a colander, reserving the brine. Rinse the cabbage with cold water and let it drain well.
Combine the ginger, garlic, apple, red pepper flakes, and fish sauce in a small blender or food processor. Blend until the ingredients form a paste.
In a large bowl, combine the cabbage, radishes, carrots, and scallions. Toss them together with your hands.
Add the seasoning paste to the cabbage mixture. Using gloved hands, massage the paste into the vegetable mixture until everything is well combined.
Begin packing your mixture into a 1/2 gallon mason jar. Use the "pickle packer" to very firmly press it down as you go. Fill the jar until it there is a 2-inch space to the top of the jar. (Use additional jars if you need to.) The kimchi mixture will begin releasing juices. If you fill the jar and don't have much there, add some of the reserved brine to just cover the vegetables.
Take the whole leaves that you had saved and cut them into large circles, the size of the opening of your jar. Place the circles (3 or 4 of them) and place them inside the jar, covering the top of the kimchi mixture.
Place a fermenting weight on top of the cabbage leaf circles, pressing the cabbage into the liquid. Top the jar with a fermentation lid.
Place the jar in small dish and put it in a cool, dark location.
In about 3 days, you should begin to see bubbling happening in the jar.
Once you see the bubbling, allow it to continue to ferment for at least 2 weeks. At that time, give it a taste and see if it tastes "done" to you. If not, let it go longer.
Once it has reached the level of fermentation that suits you, place a standard lid on the jar and store it in your refrigerator.
Enjoy!
Notes
If you are on city water that is chlorinated, simply fill a large, wide bowl with water and let it sit on the counter, uncovered. Allow it to sit several hours or overnight, and the chlorine will dissipate. Serving size suggestion is estimated at 2 Tb per serving as a condiment.