1. Preheat the oven to 375 degrees . Lightly coat a rimmed baking sheet with cooking spray. Place pistachios in a shallow dish.
2. In a small bowl, stir together mustard, orange zest, orange juice, salt and pepper. Brush mixture onto flesh side of salmon and press salmon into pistachios. Place salmon skin side down on baking sheet and bake until cooked through, about 10 minutes; transfer to a plate.
3. Preheat the broiler to high. Place onions on a clean rimmed baking sheet and drizzle with oil. Broil until brown and beginning to wilt, 2 to 4 minutes. Serve salmon with onions and, if desired, orange wedges.