1Combine white miso paste, mirin, sake, sugar, soy sauce, dashi stock, and minced ginger in a medium bowl. Whisk together until the sugar dissolves completely and the mixture becomes smooth, about 2 minutes.
2Pat black cod fillets completely dry using paper towels, removing all surface moisture. Place fillets in a shallow glass or ceramic dish in a single layer.
3Pour the miso marinade over the fish, ensuring each fillet is thoroughly coated on all sides. Reserve 3 tablespoons of marinade in a small bowl, cover, and refrigerate separately. Cover the fish with plastic wrap and refrigerate for at least 8 hours or overnight for optimal flavor development.
4Position oven rack in the middle slot (not the top) and preheat broiler to high. Line a rimmed baking sheet with aluminum foil or parchment paper for easy cleanup.
5Remove fish from refrigerator 15 minutes before cooking to take off the chill. Gently scrape off excess marinade from fillets using a butter knife or spatula, leaving only a thin coating—this prevents burning during broiling.
6Arrange fillets on the prepared baking sheet, spacing them 2 inches apart. Broil for 8-10 minutes until the miso glaze caramelizes and develops golden-brown spots on top.
7Brush reserved marinade over the fillets during the final 2 minutes of cooking for extra glaze and shine. Watch carefully to prevent burning—the fish is done when it flakes easily with a fork and reaches an internal temperature of 135°F.
8Transfer fillets immediately to warmed serving plates. Garnish with sliced green onions scattered over the top. Serve at once while the fish is hot and the glaze is glistening.