Put the miso, wine, and sugar in a resealable plastic bag and mash together until smooth. Add the fillets and marinate for in the refrigerator for at least 8 hours, or up to 2 days.
Preheat the oven to 400 F, with one rack in the middle position and one so the fish will be 4 inches from the broiler. Line a sheet pan with foil and grease with oil. Remove the fillets from the marinade and lay on the pan skin-side down.
Roast on the middle rack until rare, about 7 minutes. Remove from the oven and preheat the broiler. Flip the fillets over and broil on the upper rack until miso starts to char in spots and the fish is almost cooked through, about 2 minutes.