Pat the 2 pounds salmon dry and season both sides lightly with salt and pepper. Slice into four filets if not already sliced.
In a small bowl, whisk together 1/4 cup miso paste, 2 Tablespoons soy sauce, 1 Tablespoon brown sugar, 1 Tablespoon rice vinegar, and 1 teaspoon sesame oil until smooth.
Spoon out and set aside 1 Tablespoon of the mixture for glazing later; do not put this reserved tablespoon back into the bowl with the raw fish.
Place the salmon in a medium-sized bowl or a resealable plastic bag and pour the remaining glaze over the fish, turning to coat evenly. Cover or seal and refrigerate for 1–3 hours.
Preheat the oven to 375°F. Remove the salmon from the marinade and discard any leftover marinade from the bowl or bag.
Arrange the salmon fillets skin-side down on a baking sheet or in a baking dish. Bake at 375°F for 12–15 minutes, until the fish is mostly opaque and nearly cooked through.
Brush the top of each fillet with the reserved 1 Tablespoon glaze, then place under the broiler on high (about 4–6 inches from the heat) for 1–2 minutes, until the glaze is caramelized and the center of the fish reaches about 125–130°F.
Let the salmon rest 2–3 minutes, then garnish with chopped green onions and sesame seeds before serving.