In a small bowl, whisk together the miso paste, soy sauce, mirin, rice vinegar, and sesame oil until well combined to make the marinade.
Place the salmon fillets in a separate dish or a resealable plastic bag. Pour the miso marinade over the salmon, ensuring each fillet is evenly coated. Cover or seal the dish/bag and refrigerate for at least 30 minutes, or up to 1 hour for maximum flavor infusion.
Preheat your oven to 375°F (190°C). Lightly grease a baking dish or line it with parchment paper to prevent sticking.
Once marinated, remove the salmon from the refrigerator and transfer the fillets to the prepared baking dish, skin-side down.
Bake the salmon in the preheated oven for 12-15 minutes, or until the salmon is cooked through and flakes easily with a fork.
Remove the salmon from the oven and let it rest for a few minutes before serving.
Serve the baked miso salmon hot, garnished with sliced green onions or sesame seeds if desired.