1. In a bowl, dissolve 5 tablespoons of salt in 6 cups of water to make a brine. Set aside outer leaves from the cabbage. Cut the remaining head into quarters. Remove the core and cut the quarters crosswise to form 2-inch pieces called nabak. Soak cabbage in brine for 8 to 10 hours, or overnight.

2. Drain and rinse cabbage. Discard brine.

3. Return cabbage to bowl and add radish, scallions, ginger, red pepper flakes, garlic, and (optional) seafood ingredient.

4. Stir to combine. Pack mixture tightly into a widemouth, 2-quart canning jar with a plastic lid (metal will corrode), leaving at least 2 inches of headspace. Fold and place reserved cabbage leaves on top of kimchi to hold it down.

5. Make a fresh brine from 1 tablespoon of salt dissolved in 4 cups of water. Pour enough fresh brine to cover cabbage, leaving 1 inch of headspace. Put on the lid, making it snug but not too tight, as some gas may need to escape. Place in a bowl or on a rimmed baking sheet and put in a cool place.

6. Let ferment for 7 days. Check daily to make sure cabbage is still submerged under brine. After a week, taste, then refrigerate when you find it to be satisfyingly tart and tangy.