Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper for easy cleanup.
Make the glaze. In a small bowl, whisk together the maple syrup, Dijon mustard, olive oil, minced garlic, salt, and pepper.
Set aside about 2 tablespoons of this mixture for brushing the vegetables.
Prep the vegetables. In a separate bowl, toss the broccoli, bell pepper, and onion with 2 tablespoons of olive oil, salt, and pepper. Spread them evenly on one side of the prepared baking sheet.
Roast the vegetables first. Place the baking sheet in the oven and roast the vegetables for 10 minutes. This gives them a head start before the salmon goes in.
Prepare the salmon. While the vegetables roast, pat the salmon fillets dry with paper towels.
Season them lightly with salt and pepper.
Add the salmon to the pan. After the vegetables have roasted for 10 minutes, remove the baking sheet from the oven. Push the vegetables to one side and place the salmon fillets on the empty side. Brush the salmon generously with the maple Dijon glaze.
Bake everything together. Return the baking sheet to the oven and bake for 12 to 15 minutes, or until the salmon is cooked through and flakes easily with a fork.
The vegetables should be tender and lightly browned.
Serve immediately. Drizzle any remaining glaze over the salmon and vegetables before serving.