Simmer soy sauce, water, brown sugar, rice wine vinegar, orange juice, ginger, and garlic powder in a medium saucepan over medium heat for 3-4 minutes, stirring often, until the sugar has dissolved and mixture is well combined.
Combine cornstarch with 2 tablespoons of water in a small bowl to form the cornstarch slurry.
Increase the heat on your stovetop to medium-high. Once the teriyaki sauce has an active simmer, whisk in the slurry. It will begin to thicken fairly quickly. The sauce should easily coat the back of a spoon.
Remove the pan from the heat, cover, and keep warm until ready to use.
Preheat your grill for medium heat or 35o degrees F.
Once fully heated, oil the grill grates well using a large pair of tongs and folded paper towels dipped in oil. Make several passes to create a non-stick surface. As mentioned above, if fish does not have the skin attached, consider cooking it either on a plank or cast-iron pan.
Brush fish with oil and place it flesh side down on the grill grates. Let it cook for 2-3 minutes. The fish will naturally release when it's ready.
Use a large metal spatula to flip it onto the skin side gently.
Close the grill lid and let it cook for 2-3 minutes.
Separate the teriyaki sauce in half—Reserve half of it for serving.
Brush salmon with the other half of teriyaki sauce. Make a light coating, and apply it again a few minutes later.
Let the fish cook until it reaches an internal temperature of 145 degrees F.
Remove salmon fillets from the grill, and place them onto a platter. Drizzle with warmed reserved sauce (you can pop it in the microwave for a few seconds or rewarm briefly on the stovetop). Serve immediately.
Optional step: garnish the fish with sliced green onions and toasted sesame seeds.
*See recipe notes below for other grilling methods and oven method.