Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Pat the chicken breast dry with a paper towel to ensure the skin and herbs adhere well for a better sear.
Peel and slice the carrots and parsnips into 1/2-inch rounds, and cut the red onion into thick wedges.
In a large bowl, toss the carrots, parsnips, and onions with half of the olive oil, sea salt, and black pepper.
Spread the vegetables in a single layer on the prepared baking sheet.
Mince the garlic and finely chop the fresh rosemary and thyme.
Rub the chicken breast with the remaining olive oil, garlic, rosemary, and thyme, then place it in the center of the vegetables.
Place the lemon half cut-side down on the baking sheet to roast alongside the chicken.
Roast for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and browned.
Squeeze the juice from the roasted lemon over the entire tray before serving to brighten the flavors.