In a medium bowl, combine the softened unsalted butter, white miso paste, brown sugar, freshly grated ginger, minced garlic, and sliced green onions. Mash together until smooth.

Preheat your oven to 375°F (190°C) or air fryer to 400°F (200°C). Pat the salmon fillets dry and spread the miso butter evenly on top.

For the oven method, place the salmon in a baking dish and bake for 12-15 minutes, optionally broiling for the last 1-2 minutes.

For the air fryer method, line the basket with parchment paper, place the salmon inside, and cook at 400°F (200°C) for 9-10 minutes.

Transfer the salmon to a serving platter, drizzle with remaining miso butter and garnish with green onions. Serve with steamed rice and vegetables.