Whisk scallion, soy sauce, vinegar, honey and ginger in a medium bowl until the honey is dissolved. Place salmon in a sealable plastic bag, add 3 tablespoons of the sauce and refrigerate; let marinate for 15 minutes. Reserve the remaining sauce.
Preheat broiler. Line a small baking pan with foil and coat with cooking spray.
Transfer the salmon to the pan, skinned-side down. (Discard the marinade.) Broil the salmon 4 to 6 inches from the heat source until cooked through, 6 to 10 minutes. Drizzle with the reserved sauce and garnish with sesame seeds.