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 for a minimum of 6 hours.
Arrange the fillets on a baking sheet lined with aluminum foil. Pour marinade into a small saucepan and cook over medium-high heat until boiling. Cook for 5 minutes until reduced. Set aside.
Preheat the oven on the broiler setting and set the oven rack to be about 6 inches from the top.
Bake the sea bass under the broiler, rotate throughout cooking, until the fish flakes easily with a fork, around 7-9 minutes. Serve with marinade reduction and top with green onions.
