Combine the mirin, soy sauce, rice vinegar, sesame oil, brown sugar, garlic and ginger in a small sauce pan. Bring to a low boil, then a simmer for 10-15 minutes. Mixture will be thin and boil over easily due to the sugar, so keep the heat low and watch carefully.
Using an immersion blender or small food processor, blend well.
In a separate bowl or small measuring cup, whisk together the cornstarch and water until fully combined. Mix with teriyaki marinade until it becomes glossy. Remove from heat, mixture will thicken as it cools.
Salmon:
Preheat oven to 350°F and line a baking sheet with parchment paper.
Place salmon fillets on the baking sheet and season liberally with salt and pepper.
Bake for 10 minutes. Remove and baste with teriyaki sauce, return to the oven for 5 minutes.
Remove and baste again, return for 2 minutes.*
Remove and top with sesame seeds and scallions.
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Notes
Cooking times can vary greatly depending on the size and thickness of your salmon. Check internal temperature for a minimum of 120°F.