Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Peel and chop the carrots and parsnips into 1-inch rounds, then slice the red onion into thick wedges.
In a small bowl, whisk together the olive oil, minced garlic, finely chopped rosemary, thyme, sea salt, and black pepper.
Place the chicken breast and the prepared vegetables onto the baking sheet and drizzle with the herb oil mixture, tossing until everything is well coated.
Spread the ingredients into a single layer, ensuring the chicken is not covered by vegetables so it browns evenly.
Roast for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the root vegetables are tender and caramelized.
Allow the chicken to rest for 5 minutes before slicing it against the grain and serving alongside the roasted vegetables.