Combine the miso, rice wine vinegar, sherry, soy sauce, garlic, brown sugar, ginger, sesame seed oil, and cilantro in a non-reactive bowl. Add the black cod fillets and marinate for at least 30 minutes.
Remove the fish from the marinade, reserving the marinade. Heat a non-stick pan over medium high and add the black cod and cook for 3-4 minutes, basting occasionally. Carefully flip the fillets and cook 3-4 minutes more, or until the cod begins to flake.
Transfer to a warm plate and sprinkle with toasted sesame seeds and the green onion.