In a small bowl, whisk together white miso paste, mirin, sake, granulated sugar, light soy sauce, and grated ginger until smooth and homogenous. Taste and adjust balance: add 1 tsp more sugar if too salty, or 1 tsp more sake/mirin if too sweet. Reserve 2 tablespoons of glaze for basting after the first cook.

Marinate the fish

Pat black cod fillets dry with paper towels. Lightly score the skin once or twice at an angle (helps prevent curling). Spoon or brush 2 tablespoons of the miso glaze over the flesh side of each fillet (not the skin). Place fillets on a tray, cover, and refrigerate for 30 minutes up to 2 hours. Reserve the remaining glaze in the fridge.

Sear and broil the fillets

Preheat oven broiler to high with a rack 6 inches from the heat. Heat a large ovenproof skillet over medium-high heat with 2 tsp neutral oil. When oil shimmers, place fillets skin-side down and sear 2 minutes without moving to crisp the skin. Carefully flip, sear 30–45 seconds flesh-side just to set. Transfer skillet to the broiler and cook 4–6 minutes until glaze starts to caramelize and fish flakes easily with a fork (thicker fillets may need up to 8 minutes). Remove from oven.

Finish under broiler (optional second glaze)

If desired, brush fillets with the reserved 2 tablespoons of glaze and return to broiler 30–60 seconds just to caramelize further. Watch closely to avoid burning. Let rest 2 minutes before serving. Garnish with thinly sliced green onions.

Vegetable

While the fish is marinating, bring a large pot of salted water to a rolling boil. Blanch broccolini for 1 minute until bright green and slightly tender. Immediately transfer to an ice bath to stop cooking, then drain and pat dry.

Heat a skillet over medium heat and add 1 tbsp olive oil. Add minced garlic and sauté 20–30 seconds until fragrant (do not brown). Add blanched broccolini, a pinch of salt and black pepper, and toss 1–2 minutes until heated through and lightly coated. Finish with a squeeze of lemon if using and set aside, keeping warm.

Garlic Mashed Potatoes

Place peeled and cubed potatoes and 4 garlic cloves in a large pot and cover with cold water by 1 inch. Add 1 tbsp salt to the water. Bring to a boil over high heat, then reduce to a simmer and cook until potatoes are fork-tender, about 15–18 minutes.

While potatoes cook, gently warm milk and butter together in a small saucepan until butter melts and mixture is hot but not boiling.

Drain potatoes and garlic well and return to the hot pot. Mash using a potato masher or ricer until mostly smooth. Pour in warmed butter-milk mixture a little at a time, mashing to reach desired creaminess. Stir in sour cream if using. Season to taste with additional salt and black pepper. Keep warm until plating.

Plating and Timing

Divide garlic mashed potatoes among 4 plates (about 1/2 cup to 3/4 cup per plate depending on preference). Arrange broccolini alongside the potatoes. Place a black cod fillet on top or beside the potatoes, skin-side up to show the crisp skin. Spoon any pan juices or a small drizzle of remaining glaze over the fish (avoid flooding the plate). Garnish with sliced green onions and a lemon wedge.

Serve immediately so the fish stays flaky and the skin remains crisp.