Wipe the dried kombu with a damp paper towel to remove dirt, and make several cuts with kitchen scissors. Place the kombu and water in a pot and let it sit for 30 minutes.

Cut the napa cabbage into bite-sized pieces. Slice the carrot diagonally into 5mm pieces. Cut the green onion into 1cm diagonal slices. Remove the tough stems of the shiitake mushrooms, and make cross cuts on the caps for decoration.

In a bowl, mix the oysters with 2 tablespoons of potato starch (not listed in ingredients) and gently rub. Rinse with water until clear and drain well.

Heat the pot with kombu over low heat. Remove the kombu just before boiling. Add napa cabbage, carrot, green onion, and shiitake mushrooms. Cover and simmer for 5 minutes.