Prepare a 10-inch pot or dutch oven.
Cut the cabbage into quarters lengthwise. Do not remove the bottom. Carefully wash each quarter under running water and keep the whole thing intact. Set aside and let it drain. Continue with the other quarters. Reserve any leaves that got washed off.
In a medium pot over medium-heat, add 6 cups water and 2 dashi packets. Bring to a boil and simmer for 5 minutes. Discard the packets and remove from heat. Set aside.
Back to the cabbage, lay a slice of pork belly in between the cabbage leaves. If the leave is big, you can put 2 slices of pork belly side-by-side. After the cabbage is all stuffed, trim off the cabbage bottom. While holding the cabbage quarter tight together, carefully cut the quarter lenthwise into 2½-inch pieces.
In the prepared pot, starting from the outer edge, place the cabbage & pork layers. Make sure everything is packed tightly. If needed, put some of reserved leaves in the middle.
Measure out 5 cups of dashi broth (not all) to a large bowl. Add sake, soy sauce and salt to the bowl.
Place the ginger slice on top of the cabbage and pour in the dashi mixture.
Cook over high heat and bring to a boil. Once boiling, reduce heat to medium-low and cook until cabbage is tender and meat is cooked through, about 5 – 8 minutes. Sprinkle with chopped green onion. Serve with ponzu sauce and shichimi togarshi.