Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Peel and chop the carrots and parsnips into 1-inch chunks, and slice the red onion into thick wedges.
In a mixing bowl, toss the vegetables with half of the olive oil and a pinch of sea salt until evenly coated.
Spread the vegetables in a single layer on the prepared baking sheet, leaving space in the center for the chicken.
Mince the garlic and finely chop the fresh rosemary.
Pat the chicken breast dry with a paper towel, then rub it with the remaining olive oil, minced garlic, rosemary, sea salt, and black pepper.
Place the chicken breast in the center of the baking sheet.
Roast 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 to keep it juicy.
Serve the sliced chicken alongside the roasted root vegetables.