Preheat oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Pat the chicken breast dry with a paper towel and slice into even 1-inch pieces.
Peel and chop the carrots and parsnips into 1/2-inch rounds or half-moons.
In a large bowl, whisk together the olive oil, minced garlic, rosemary, thyme, salt, and pepper.
Add the chicken, carrots, parsnips, and onions to the bowl and toss until everything is thoroughly coated in the herb oil.
Spread the mixture in a single layer on the prepared sheet pan, ensuring pieces aren't overcrowded.
Roast for 20-25 minutes, tossing halfway through, until the chicken is cooked through and the vegetables are tender and golden brown.