Rinse the bok choy and cut it into strips. Cut the scallion into fine rings and grate the fresh ginger. Pat the cod fillet dry with paper towels.
Warm the dashi or vegetable broth in a pan over medium-high heat. Add the white miso paste and stir well until the paste is completely dissolved. Mix the grated ginger through and bring the broth gently to a simmer, without letting the liquid boil vigorously.
Place the cod fillet in the hot broth and poach the fish for a total of five minutes. After three minutes, add the bok choy strips to the pan, so they can gently simmer for the last two minutes. Remove the pan from the heat once the fish flakes apart beautifully and the vegetables have slightly wilted.
Carefully scoop the cod and bok choy from the pan and place them in the center of a deep bowl. Pour the warm miso broth over them. Garnish the dish with the scallion rings, sesame seeds and sesame oil.