Heat oven to 450° F, with the racks in the upper and lower thirds. On a rimmed baking sheet, toss the Brussels sprouts and onion with the oil, ½ teaspoon salt, and ¼ teaspoon pepper. Roast on the bottom rack, tossing once, until golden and tender, 15 to 20 minutes.
Meanwhile, in a small bowl, mix together the maple syrup, mustard, and ¼ teaspoon each salt and pepper. After the vegetables have cooked for 12 minutes, place the salmon on a second rimmed baking sheet and brush it with the maple mixture. Roast on the top rack until opaque throughout, 6 to 8 minutes.
Serve the salmon with the vegetables and lemon wedges.