Preheat the oven's broiler and set the oven rack at about 6 inches from the heat source; spray the rack of a broiler pan with cooking spray.
Season the salmon with salt and pepper and arrange onto the broiler pan. Whisk together the brown sugar and Dijon mustard in a small bowl then spoon mixture evenly onto top of salmon fillets.
Cook under the preheated broiler until the fish flakes easily with a fork, 10 to 15 minutes.
