Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or foil.
- In a bowl, toss carrots, parsnips, sweet potato (or squash), and red onion with olive oil, salt, pepper, and rosemary if using. Spread evenly on the baking sheet and roast for 20 minutes.
- In a small bowl, whisk together maple syrup, Dijon mustard, garlic, lemon juice, olive oil, salt, and pepper to make the glaze.
- Remove vegetables from the oven. Push them to the sides of the pan and place salmon fillets in the center, skin-side down if applicable. Brush generously with glaze.
- Return to the oven and roast 12–15 minutes, until salmon flakes easily and vegetables are tender. Broil for 1–2 minutes at the end for extra caramelization if desired.
- Serve hot, garnished with fresh thyme or parsley and a squeeze of lemon juice.