Heat the oven. Set your oven to 425°F (220°C). Line a large sheet pan with foil or parchment for easy cleanup.
Prep the sprouts. Trim the ends and halve any large Brussels sprouts. Toss them on the sheet pan with 2 tablespoons olive oil, 1/2 teaspoon salt, and several grinds of pepper.
Spread them cut-side down for maximum browning.
Roast the sprouts first. Roast for 12 minutes to jump-start crisping. They should be beginning to brown on the edges.
Mix the glaze. In a small bowl, combine maple syrup, soy sauce, Dijon, garlic, apple cider vinegar, 1 tablespoon olive oil, and a pinch of red pepper flakes if using. Taste and adjust salt and acidity as needed.
Season the salmon. Pat the fillets dry and sprinkle both sides with salt and pepper.
Brush a little of the glaze over the top of each fillet.
Add salmon to the pan. Pull the sprouts from the oven, toss them, and push them to the sides. Place the salmon in the center, skin-side down. Spoon more glaze over the salmon and drizzle a spoonful over the sprouts.
Roast to finish. Return the pan to the oven and roast 8–12 minutes, depending on fillet thickness.
Salmon is done when it flakes easily and reaches 125–130°F for medium (or your preferred doneness).
Broil for a hint of char (optional). For extra caramelization, switch to broil for 1–2 minutes. Watch closely to avoid burning.
Serve. Squeeze fresh lemon over everything, add any remaining glaze from the bowl, and sprinkle with herbs or nuts if you like. Serve hot.