For the marinade: In a microwave-safe bowl, mix together miso paste, mirin, sake and brown sugar.
Cook marinade in the microwave for 2 minutes until most of the sugar is dissolved and mix to combine the ingredients.
Add the cod to the bowl and rub marinade over both sides, if possible let the cod sit in the marinade between 5-30 minutes (preferably closer to 30 minutes).
Place cod fillets on a sheet pan lined with foil and roast the cod fillets in the oven for 8- 9 minutes until fillets are fork tender and caramelized.
Recipe by Momma Chef at https://mommachef.com/miso-glazed-cod-easy-quick-recipe-momma-chef/