Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Prepare the vegetables by trimming and halving the Brussels sprouts, peeling and slicing carrots into 1/2-inch rounds, and cutting the red onion into thick wedges.
Place the chicken breast in the center of the baking sheet and surround it with the prepared vegetables in a single layer.
Drizzle the olive oil evenly over the chicken and the vegetables.
Whisk the dried rosemary, thyme, garlic powder, sea salt, and black pepper together in a small bowl, then sprinkle the mixture over everything on the tray.
Toss the vegetables with your hands or tongs to ensure they are well-coated, and rub the seasoning into both sides of the chicken.
Roast in the center of the oven for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with caramelized edges.
Remove from the oven and let the chicken rest for 5 minutes before slicing it against the grain to maintain juiciness.
Plate the sliced chicken alongside the roasted vegetables and serve immediately.