Preheat the oven to 400°F and line a baking sheet with parchment paper or foil.
In a small bowl, whisk together the miso paste, mirin, soy sauce, granulated sugar, grated fresh ginger, and minced garlic until smooth.
Pat the salmon fillets dry with a paper towel and place them on the prepared baking sheet.
Spoon the miso mixture evenly over each salmon fillet, spreading to coat the tops. (Optional: let sit at room temperature for 15–20 minutes to marinate.)
Bake for 12–15 minutes, or until the salmon reaches an internal temperature of 145°F and flakes easily with a fork.
Switch the oven to broil and broil for 1–2 minutes, watching closely, until the glaze is golden and slightly bubbly.
Remove from the oven, drizzle with sesame oil if desired, and garnish with sliced green onions before serving.