Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Prepare the vegetables by dicing the sweet potato and carrots into 1/2-inch cubes and halving the Brussels sprouts.
Mince the garlic cloves and finely chop the fresh rosemary leaves.
In a large mixing bowl, combine the chicken breast, sweet potatoes, carrots, and Brussels sprouts.
Drizzle with olive oil and sprinkle with the minced garlic, rosemary, sea salt, and black pepper, tossing well to ensure everything is evenly coated.
Spread the mixture in a single layer on the baking sheet, ensuring the chicken and vegetables are not crowded so they roast rather than steam.
Roast for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the root vegetables are tender and golden brown.
Remove from the oven and let the chicken rest for 5 minutes before slicing to retain its juices, then serve immediately.