Boil saké and mirin together for a few seconds,
Turn heat to low and stir in miso paste.
Turn heat back up to medium and add sugar, stirring until it is all dissolved.
Cool miso mixture to room temperature.
Use this mixture to marinate sablefish fillets for 1 to 3 days, reserving 1/4 cup for garnish.
To cook, remove fish from marinade and gently wipe or shake excess away, but don't rinse.
Broil or grill your marinated sablefish until the surface is golden brown, then bake for 10 minutes.
Serve each portion over a handful of baby spinach greens and garnish with a little of the extra miso mixture.