In a medium-size bowl, combine softened butter, miso paste, brown sugar, grated ginger, minced garlic, and sliced green onions. Use a spatula to mash everything together until well blended and creamy.
Equally divide the miso butter mixture on top of each salmon fillet. Using a butter knife or an offset spatula, spread the compound butter over the entire surface of each piece of salmon for maximum flavor.
Preheat your oven to 375°F (190°C). In a large skillet or baking dish, place the salmon fillets inside and bake them for 12-15 minutes until they are opaque and nearly cooked through. To finish off, broil on high for 1-2 minutes until the tops become beautifully caramelized.
If you prefer using an air fryer, preheat it to 400°F (200°C). Line the air fryer basket with parchment paper or foil before placing the salmon fillets inside. Cook for approximately 9-10 minutes without turning them over; they will be perfectly cooked and tender.
Once cooked, transfer the salmon onto a serving platter. Drizzle any remaining miso butter on top of the fillets and garnish with additional green onions and sesame seeds if desired. Serve alongside steamed rice and vegetables for a complete meal.