Place soy sauce, brown sugar, ground ginger, garlic powder, honey, sesame oil and sherry in a small saucepan and whisk to combine.
Set over medium heat until sugar is completely melted and no longer grainy, about 4 minutes.
Whisk the cornstarch into the water and then add to the sauce mixture. Continue to cook for 4 minutes until sauce is thick enough to coat the back of a spoon.
Place the salmon portions in a small bowl or casserole dish and cover with about 1/2 of the teriyaki sauce, reserving the remaining half for later.
Refrigerate salmon for 30 minutes to marinate.
Preheat oven to 400F.
Line a baking sheet with parchment paper and set the marinated salmon on the baking sheet. Brush with additional teriyaki sauce, as needed.
Bake for 14-16 minutes until an inserted thermometer reaches 125F-135F.
Remove salmon from oven and brush with additional teriyaki sauce, if desired.
Serve with sliced green onions.