Combine 1/4 cup cold water and cornstarch in a small bowl. Seal and shake until cornstarch dissolves; set aside.
STEP 2
Combine remaining 1 cup water, soy sauce, brown sugar, garlic, and sesame oil in a saucepan over medium-high heat; bring to a boil. Reduce heat to a simmer; stir in cornstarch mixture. Simmer until sauce thickens, about 5 minutes. Remove from heat. Reserve 1/2 cup glaze; set aside.
STEP 3
Set oven rack about 6 inches from the heat source and preheat broiler to low. Place salmon on a foil-lined baking sheet. Brush all sides with the remaining teriyaki glaze.
STEP 4
Broil salmon for 12 minutes; brush reserved 1/2 cup glaze over top. Turn broiler to high; broil until salmon flakes easily with a fork, about 3 minutes. Garnish with green onion and sesame seeds to serve.