Whisk together the maple syrup and mustard. Season both sides of the salmon fillets liberally with salt and pepper, then brush them with the maple-mustard mixture and set aside.
Meanwhile, heat the oven to 450°F. In a large bowl, toss the sprouts and onion with the olive oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until well-coated. Spread them out on a baking sheet, cut side-down, and roast on the lower rack until caramelized and tender, 20-30 minutes.
When the sprouts are nearly done, arrange the fillets on an oven-proof dish and broil them, skin-side down, until the maple mixture is well-caramelized and the fish is just cooked through—7 to 10 minutes, depending on the thickness of the fillet.
Serve with the sprouts and lemon wedges.