Preheat oven to 425°F (220°C). In a large bowl, whisk together maple syrup, oil, garlic, lemon juice, mustard, salt and pepper. Pour 1/4 cup (60 mL) of the maple mixture into a small bowl and set aside.
Line a large rimmed baking sheet with parchment paper or foil. Add Brussels sprouts and onion to large bowl with maple mixture and gently toss. Spread Brussels sprout mixture on prepared baking sheet. Place on middle rack of oven and roast, stirring once halfway through, until tender and golden, about 20 min.
Remove baking sheet from oven and turn oven to broil. Sprinkle raisins onto Brussels sprout mixture, place salmon overtop and brush with reserved maple mixture. Place baking sheet back in oven and broil until salmon is slightly golden on top and flakes easily with fork, about 7 min. Sprinkle with parsley before serving.
Give your vegetables ample space on the baking sheet to ensure even roasting rather than steaming.