Preheat oven grill.
1) Rinse fish fillets and pat dry with kitchen paper. Combine miso, brown sugar, sesame oil and mirin and stir well until brown sugar is fully dissolved.
2) Brush about two tablespoons miso glaze on each fish fillet. Marinate for at least 30 minutes, or up to one hour. Place fish under grill for 3 to 4 minutes, or until top is slightly charred and glaze has caramelised.
3) Remove fish from oven and brush with remaining glaze. Lower oven to 190°C/ gas mark 5. Cook for an additional 5 to 6 minutes, until fish is flaky but not overcooked.
If desired, serve with toasted sesame seeds and spring onions.