Whisk together the sake, mirin, soy sauce, brown sugar and miso paste in a bowl to make the marinade. Place the sea bass in a large sealable plastic bag and pour the marinade over the sea bass. Chill in refrigerator 3 to 6 hours. Arrange the fillets on a baking tray. Discard the marinade.
Preheat the grill and set the oven rack about 15cm from the heat source. Prop the oven door to remain slightly ajar.
Bake the sea bass under the grill until the fish flakes easily with a fork, 7 to 9 minutes. Sprinkle with chopped spring onions to serve.