In a small bowl, whisk together the white miso paste, mirin, tamari, and grated ginger until the mixture is smooth.
Pat the black cod fillet dry with a paper towel and coat all sides thoroughly with the miso glaze.
Place the fish in a shallow dish and allow it to marinate in the refrigerator for at least 15 to 30 minutes.
Preheat your oven broiler to high and line a baking sheet with parchment paper or lightly grease a broiler pan.
Arrange the cod on the prepared pan and broil for 6 to 8 minutes until the top is deeply caramelized and the fish flakes easily.
While the fish is broiling, heat the toasted sesame oil in a large skillet over medium heat.
Add the minced garlic and sauté for 30 seconds until fragrant, then toss in the baby bok choy.
Season the vegetables with sea salt and black pepper, sautéing for 3 to 4 minutes until the leaves are wilted and the stems are tender.
Plate the caramelized black cod immediately alongside the warm bok choy.