Preheat your oven to 400°F (200°C) and line a baking dish with parchment paper or lightly oil it to prevent sticking.
Make the teriyaki sauce by combining soy sauce, mirin, brown sugar, rice vinegar, garlic, and ginger in a small saucepan over medium heat. Bring to a simmer and cook for 3-5 minutes until the sugar has completely dissolved.
Add the cornstarch slurry to the simmering sauce, whisking constantly until the sauce thickens enough to coat the back of a spoon, about 1-2 minutes. Remove from heat and let cool slightly.
Pat the salmon fillets dry with paper towels (this ensures better caramelization) and place them skin-side down in the prepared baking dish, leaving a little space between each fillet.
Reserve 1/4 cup of the teriyaki sauce for serving, then brush the remaining sauce generously over the salmon fillets, making sure to coat the tops and sides well.
Bake in the preheated oven for 15-18 minutes, or until the salmon flakes easily with a fork but is still moist in the center. The exact cooking time will depend on the thickness of your fillets.
Brush the salmon with more teriyaki sauce halfway through cooking to build up that delicious glaze.
For extra caramelization, broil on high for the last 1-2 minutes, watching carefully to avoid burning.
Let the salmon rest for 3-5 minutes after removing from the oven. This allows the juices to redistribute throughout the fish.
Drizzle with the reserved sauce, then garnish with sliced green onions and sesame seeds before serving.