1) Make the miso marinade
In a small saucepan over medium heat, whisk together the mirin, sake, and sugar. Bring to a gentle simmer and whisk until the sugar dissolves, about 1 to 2 minutes.
Turn the heat to low. Add the miso and whisk until smooth. If using soy sauce, whisk it in now. Take off the heat and let the marinade cool to room temp.
2) Marinate the fish
Pat the black cod dry. Spread a thin layer of cooled marinade in a shallow dish. Nestle the fish in, then spoon the rest of the marinade over the top to coat.
Cover and refrigerate at least 6 hours, ideally 24 to 48 hours.
3) Prep to broil
When you are ready to cook, set your oven rack about 6 inches from the broiler and preheat the broiler on high.
Line a rimmed baking sheet with foil. Lightly oil the foil or a wire rack set on the sheet.
Remove the fish from the marinade and scrape or wipe off most of the excess so only a thin film remains. Thick globs burn before the fish is cooked. Discard the used marinade.
Skin-on note: If your fillets have skin, broil skin-side down. The skin acts like a little shield and helps keep the fish extra moist.
4) Broil until caramelized
Place the fish on the prepared sheet pan. Broil 6 to 8 minutes, depending on thickness, until the top is caramelized and the fish is opaque and separates into large flakes with gentle pressure. Sablefish stays soft even when cooked, so go by opacity and gentle flaking, not “dry and flaky.”
Broilers vary a lot, so start checking at 4 minutes. Rotate the pan halfway through if your broiler has hot spots.
Temperature guide: For a silky, just-cooked finish, many cooks pull it around 130°F to 135°F in the thickest part and let it rest. For an FDA-style well-done target, cook to 145°F.
Rest 2 minutes, then serve with rice and a green vegetable. Finish with scallions, sesame seeds, and a squeeze of lemon if you want a brighter pop.
5) Serve
Slide a fillet onto a plate, spoon any glaze from the pan over the top, and build your plate around it: a mound of rice, a pile of greens, and a little lemon on the side. Minimal effort, maximum payoff.
