Rinse the napa cabbage thoroughly, set aside one intact outer leaf, then cut the remaining cabbage into 1-inch pieces and place it in a very large bowl. Keep the reserved leaf clean and whole because it will act as a natural cover in the jars.

Scatter the coarse kosher salt over the cabbage, pour in enough water to cover it, then cover the bowl with plastic wrap and leave it overnight. The leaves should soften and bend easily while retaining a crisp core.

Blend the fish sauce, gochugaru, garlic, and ginger in a small food processor or with a stick blender until thick and evenly combined. The paste should be vivid red and dense enough to coat the cabbage rather than run off it.

Lift the cabbage from the brine and squeeze the liquid back into the bowl, then transfer the cabbage to a medium mixing bowl with the scallions cut into 1-inch pieces. Remove excess water from the leaves while saving the salty liquid for the jars.

Add the chili paste and massage it into every fold of cabbage with gloved hands. The leaves should become evenly red, glossy, and strongly aromatic.

Divide the seasoned cabbage between two quart-sized mason jars and press it down firmly to remove trapped air pockets. Leave the cabbage compact rather than loosely piled so the brine can rise around it.

Pour in the reserved brine until the cabbage is fully covered, split the reserved leaf between the jars, and press it over the top before fermenting in a cool, dark room for 3 to 5 days. Look for active bubbles along the jar sides, then refrigerate when the kimchi is lightly sour and still pleasantly salty.