In a medium bowl, combine soy sauce, brown sugar, minced garlic, and grated ginger. Whisk until the ingredients are thoroughly mixed, creating a fragrant marinade.

Place the black cod fillets in a shallow dish and pour the marinade over them. Ensure the fillets are well-coated and refrigerate for at least 30 minutes.

Preheat your oven to 400°F (200°C).

Heat the sesame oil in an oven-safe skillet over medium-high heat. Add the marinated fillets skin-side down and sear for 4-5 minutes until golden brown.

Transfer the skillet to your preheated oven and bake the fillets for an additional 8-10 minutes until the fish flakes easily with a fork.

Remove the skillet from the oven and sprinkle with sliced green onions before serving.