1 lb Brussels sprout, trimmed and halved
1 large red onion, cut into 1-inch wedges, stem ends left intact
2 tablespoons olive oil
2 tablespoons pure maple syrup
2 tablespoons whole grain mustard
1 12 lbs skinless salmon fillet, cut into 4 pieces
lemon wedge, for serving
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, 1/2 teaspoon salt, and 1/4 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 1/4 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.