In a medium saucepan, combine water, dark brown sugar, reduced-sodium soy sauce, honey, rice vinegar, minced garlic, fresh ground ginger, and red pepper flakes. Heat over medium heat and stir for about 5-7 minutes until combined, then remove from heat to cool.

In a small bowl, mix cornstarch with cold water until smooth. Slowly add this mixture to the cooled teriyaki sauce while stirring constantly over low heat for 2-3 minutes until thickened.

Place salmon fillets in a shallow bowl. Pour the cooled teriyaki sauce over the salmon, cover with plastic wrap, and refrigerate for at least 20 minutes.

Preheat your oven to 400°F (200°C) and prepare a greased baking sheet or line it with parchment paper.

Remove the salmon from the refrigerator, place the fillets on the prepared baking sheet skin side down, and discard any remaining marinade. Bake for 12-16 minutes or until the salmon flakes easily.

Simmer the remaining marinade in a small saucepan for about 5 minutes, then brush this glaze over the cooked salmon. Garnish with chopped green onions and sesame seeds before serving.