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 chunks, then trim and halve the Brussels sprouts.
Cut the chicken breast into uniform 1-inch pieces to ensure they cook at the same rate as the root vegetables.
In a large mixing bowl, combine the chicken and vegetables with the olive oil, rosemary, garlic powder, sea salt, and black pepper.
Toss everything thoroughly until the chicken and produce are well-coated in the herb oil.
Spread the mixture in a single layer on the prepared baking sheet, making sure not to crowd the pan so the vegetables can crisp properly.
Roast for 20 to 25 minutes, tossing the ingredients halfway through, until the chicken is cooked through and the vegetables are tender and golden brown.