Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper.
Peel and chop the carrots and parsnips into uniform 1-inch chunks to ensure they cook at the same rate.
Mince the garlic cloves and finely chop the fresh rosemary and thyme leaves, discarding the woody stems.
In a large mixing bowl, combine the chicken breast, chopped root vegetables, olive oil, minced garlic, herbs, salt, and pepper.
Toss everything thoroughly until the chicken and vegetables are evenly coated in the oil and aromatics.
Spread the mixture onto the prepared sheet pan in a single layer, making sure not to crowd the ingredients so they roast rather than steam.
Roast for 20 to 25 minutes, or until the chicken is cooked through to an internal temperature of 165°F and the vegetables are tender and lightly browned.
Remove from the oven and let the chicken rest for 5 minutes before slicing to ensure it stays juicy, then serve immediately.