In a mixing bowl, combine 3 tablespoons of miso paste, 2 tablespoons of brown sugar, 1 tablespoon of rice vinegar, 1 tablespoon of sesame oil, and 1 minced garlic clove. Stir until smooth.

Gently dab cod loins with paper towels and coat generously with the miso marinade. Refrigerate for at least 1 hour.

Heat a non-stick frying pan over medium heat and add 3 tablespoons of vegetable oil until it glistens.

Cook the marinated cod skin-side down for 3-4 minutes, then pour any excess marinade over and cover for an additional 3 minutes.

Flip the cod over, increase heat, and drizzle remaining marinade over the fish. Cook for an additional minute.

Remove from heat and garnish with freshly sliced spring onions. Serve immediately.