Sprinkle some salt over the oysters and add some water to gently rub the oysters to clean in a bowl. Wash out the salt under running water. Strain water and set aside.
Cut Nappa cabbage into bite-size pieces and slice green onions diagonally. Cut the Shiitake mushroom in half and cut tofu into bite-size pieces. Separate shungiku greens from their stems.
Put the ingredients [A] in a Japanese clay pot. Heat over low heat and mix them with a spatula. Heat until the water has evaporated and the mixture is soft enough not to fall off when scooped with a spatula. Remove the pot from the heat and smear the Miso mixture on the inner wall of the pot so that it forms a bank.
Place a nappa cabbage core in the pot’s bottom and lay all the other vegetables and mushrooms except the shungiku leaves on top. Addnwater and heat over medium heat. Bring to a boil and put oysters on top. Once all ingredients are cooked, add the remaining shungiku leaves.