Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Pat the chicken breast dry with paper towels and season both sides with half of the sea salt and black pepper.
Peel and chop the carrots and parsnips into 1-inch chunks, and cut the red onion into thick wedges.
In a large bowl, toss the carrots, parsnips, and onion with the extra virgin olive oil, minced garlic, chopped rosemary, and the remaining salt and pepper.
Spread the vegetables in a single layer on the prepared baking sheet, leaving an open space in the center.
Place the seasoned chicken breast in the center of the sheet pan.
Roast for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are golden and tender.
Remove from the oven and let the chicken rest for 5 minutes before slicing.
Serve the sliced chicken alongside the roasted root vegetables, drizzling any remaining pan juices over the top.