Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Slice the carrots into thin rounds, the zucchini into half-moons, and the red onion into thick wedges.
Pat the chicken breast dry with a paper towel and cut into 1-inch cubes to ensure even cooking.
In a large mixing bowl, combine the chicken, carrots, zucchini, and onion.
Mince the garlic and finely chop the fresh rosemary, then add them to the bowl.
Drizzle with olive oil and sprinkle with sea salt and black pepper, tossing well to coat every piece.
Spread the mixture in a single layer on the prepared baking sheet, ensuring nothing is overcrowded.
Roast for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender and lightly caramelized.