Mix miso, mirin, sake, sugar, and soy sauce until sugar dissolves
Marinate fish in miso mixture for at least 2 hours (or overnight)
Preheat broiler to high
Remove fish from marinade, scraping off excess
Place on a foil-lined baking sheet
Broil for 8-10 minutes until caramelized and cooked through
Watch carefully to prevent burning
Garnish with green onions and serve