Pat the salmon fillets dry with paper towels and place them on a plate or tray.
In a small mixing bowl, whisk together miso paste, soy sauce, mirin, sugar, and rice vinegar until well combined and smooth.
Using a brush or spoon, evenly spread the miso marinade over both sides of each salmon fillet, ensuring full coverage.
Preheat your oven to 400°F (200°C). Line a baking sheet with aluminum foil or parchment paper and lightly brush with vegetable oil.
Place the marinated salmon fillets on the prepared baking sheet, skin-side down if skin is present, and bake for 15-20 minutes, until the surface is caramelized and the fish is opaque and flaky.
Remove the salmon from the oven, transfer to a serving plate, and garnish with a sprinkle of thinly sliced green onions if desired. Serve hot with your preferred side.