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.
How to skin a salmon fillet: Place salmon fillet on a clean cutting board, skin-side down. Starting at the tail end, slip the blade of a long knife between the fish flesh and the skin, holding down firmly with your other hand.
Gently push the blade along at a 30° angle, separating the fillet from the skin without cutting through either.
To toast seeds: Cook in a small dry skillet over medium-low heat, stirring constantly, until fragrant and lightly browned, 2 to 4 minutes.
