Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Peel and chop the carrots and parsnips into one-inch pieces, and slice the red onion into wedges.
In a large bowl, toss the chopped root vegetables and onion with half of the olive oil, sea salt, and black pepper until evenly coated.
Spread the vegetables in a single layer on the prepared baking sheet.
Mince the garlic and finely chop the fresh rosemary, then rub them onto the chicken breast along with the remaining olive oil.
Nestle the seasoned chicken breast among the vegetables on the baking sheet.
Roast for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and browned.
Remove from the oven and allow the chicken to rest for 5 minutes before slicing to ensure the juices remain locked in.
Serve the sliced chicken alongside the roasted root vegetables for a complete, clean-eating meal.