In a small bowl, whisk together the white miso paste, mirin, sake, sugar, grated ginger, and soy sauce until the mixture is uniform and smooth.
Pat the salmon fillets dry with paper towels to ensure proper glaze adhesion.
Coat the salmon fillets thoroughly with the miso marinade in a shallow dish or resealable plastic bag.
Allow the salmon to marinate in the refrigerator for at least 30 minutes.
Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
Place the salmon fillets on the baking sheet, skin-side down, and wipe away any excessive pools of marinade to prevent burning.
Bake for 10 to 12 minutes, or until the fish flakes easily with a fork and the internal temperature reaches 145°F (63°C).
Garnish with sliced scallions and serve immediately.