Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Peel the carrots and parsnip, then dice them into 1-inch chunks; slice the red onion into thick wedges.
Mince the garlic cloves and finely chop the fresh rosemary leaves.
In a large mixing bowl, combine the chicken breast, carrots, parsnips, and onion.
Drizzle the olive oil over the chicken and vegetables, then add the minced garlic, rosemary, sea salt, and black pepper.
Toss everything thoroughly until the chicken and vegetables are evenly coated in the oil and seasonings.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the chicken pieces are not crowded.
Roast in the center of the oven for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and golden.
Remove from the oven and let the chicken rest for 5 minutes before slicing and serving with the roasted roots.