In a small mixing bowl, combine 2 tablespoons of miso paste, 1 tablespoon of brown sugar, 1 tablespoon of rice vinegar, 1 teaspoon of sesame oil, and one minced garlic clove. Blend until smooth and fragrant.

Pat the cod loins dry with paper towels. Place cod in a shallow dish and pour the marinade over the top, covering evenly. Cover with plastic wrap and refrigerate for at least 1 hour.

Heat 3 tablespoons of vegetable oil in a large non-stick frying pan over medium heat for about 2 minutes until it shimmers.

Place the marinated cod in the hot pan, skin side down. Let it cook undisturbed for 3 to 4 minutes.

Cover the pan with a lid and continue cooking for another 3 minutes.

Remove the lid, gently flip the cod, increase heat to high, and cook for an additional minute. Spoon remaining marinade over cod to glaze.

Check that cod is cooked through (internal temperature of 145°F). Remove from pan, transfer to a plate, and garnish with sliced spring onions.