In a medium bowl, combine softened unsalted butter, white miso paste, brown sugar, grated fresh ginger, and minced garlic. Mix until smooth and well blended.

Preheat your oven to 375°F (190°C) for baking or 400°F (200°C) for air frying. Place the salmon fillets on a parchment-lined baking dish or air fryer basket.

Spread a generous layer of the miso butter mixture over each salmon fillet, ensuring all surfaces are covered.

For the oven, bake the salmon for 12-15 minutes until opaque; for air fryer, cook for 9-10 minutes without flipping.

If using the oven, broil for the last 1-2 minutes for a caramelized top. Watch closely to avoid burning.

Once cooked, transfer the salmon to a serving platter, drizzle with remaining miso butter, and garnish with sliced green onions.