Mille-Feuille Nabe (Napa Cabbage and Pork Hot Pot)

Steps

1. Make the dashi: put the kombu in the cold water in a saucepan and let it soak for 20 minutes. Then heat gently for about 10 minutes, until tiny bubbles form around the kombu, around 175°F (80°C). Remove the kombu before the water boils, or the broth turns slimy and bitter.
2. Bring the water to a boil, turn off the heat, and add the bonito flakes all at once. Let them steep for 3 minutes, until they sink to the bottom, then strain through a fine sieve lined with a paper towel, without pressing.
3. Pull off the 2 or 3 damaged outer leaves of the napa cabbage, trim the base, and separate all the leaves. Wash and drain them well. Peel the ginger and cut it into fine matchsticks.
4. Build the stacks: lay one large cabbage leaf flat, cover its whole surface with pork slices, scatter a few ginger matchsticks over it, then add another cabbage leaf. Make stacks of 4 cabbage leaves and 3 layers of pork, ending with cabbage, alternating the direction of the leaves so the thick ribs aren't all on the same side.
5. Cut each stack into 2 inch (5 cm) wide sections. The sections should be a little shorter than the depth of your pot.
6. Stand the sections on their cut edge, layers facing up, packing them tightly against each other in a circle from the edge toward the center of a donabe or a 9 1/2 inch (24 cm) pot. Press them in firmly: the cabbage shrinks by half as it cooks and the rosette mustn't collapse. Leave a small space in the center.
7. Remove the shiitake stems and cut a cross into each cap. Tuck them into the center of the rosette with the rest of the ginger.
8. Mix the dashi with the sake, soy sauce, and salt. Pour it gently down the side of the pot until it comes two-thirds of the way up the cabbage: the cabbage will release its own water as it cooks.
9. Cover and heat over medium. With a clay donabe, raise the heat gradually and make sure the bottom is completely dry. As soon as it boils, lower to a simmer and cook for 15 to 20 minutes: the pork should be fully white with no trace of pink, at least 145°F (63°C), and the tip of a knife should slide through the cabbage ribs with no resistance.
10. Thinly slice the green onions and scatter them in the center of the rosette. Bring the pot to the table, on a tabletop burner if you have one. Pour the ponzu into individual dipping bowls: everyone takes a section of cabbage and pork, dips it in ponzu, and sips a little broth.