In a medium bowl, whisk together the miso paste, mirin, sugar, soy sauce, sesame oil, and grated ginger until smooth.
Place the salmon fillets in a shallow dish and pour the miso marinade over them, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Remove the salmon from the marinade, allowing any excess to drip off. Place the fillets skin-side down on the prepared baking sheet.
Bake the salmon in the preheated oven for 12-15 minutes, or until the fish flakes easily with a fork and is cooked through.
Optional: For a caramelized finish, broil the salmon for an additional 2-3 minutes at the end of cooking.
Remove from the oven and let it rest for a couple of minutes. Garnish with sliced green onions and sesame seeds before serving.