Whisk together the white miso, mirin, sake, sugar, and soy sauce in a small bowl until smooth and the sugar has dissolved.

Place the black cod fillets in a shallow dish or zip-lock bag and coat each fillet evenly with the marinade.

Cover and refrigerate for at least 24 hours, or up to 48 hours for maximum flavor.

Remove the fillets from the marinade and let them sit at room temperature for 15 minutes. Heat the oven broiler to its highest setting (about 230 C / 450 F).

Lightly oil a wire rack set over a foil-lined rimmed baking sheet.

Use a paper towel to gently wipe most of the excess marinade off each fillet, leaving a thin even coating.

Place fillets skin-side down on the prepared rack. Broil on the top rack for 8-10 minutes, until the glaze is deep golden-brown and caramelized and the fish flakes easily at the thickest point (internal temperature 60 C / 140 F).

If the glaze darkens too fast before the fish is cooked through, move the rack one level lower and continue broiling.

Heat 1 tbsp neutral oil in a 12-inch nonstick skillet over medium-high heat.

Add the garlic slices and cook for 30 seconds, stirring, until fragrant but not browned.

Add the bok choy halves cut-side down. Let them sear undisturbed for 2 minutes until lightly golden on the cut surface.

Add the soy sauce and water. Cover with a lid and steam for 2-3 minutes until the stems are just tender when pierced with a knife tip.

Remove from heat, drizzle with toasted sesame oil, and season with a pinch of kosher salt.

Place a portion of steamed rice in the center of each plate if using.

Set one black cod fillet alongside or on top of the rice.

Arrange two bok choy halves beside the fish. Garnish with sesame seeds and scallions, and serve with a lemon wedge.