Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Prepare the vegetables by trimming the ends off the Brussels sprouts and slicing them in half lengthwise.
Peel the carrots and slice them into 1-inch thick rounds, and chop the red onion into large, bite-sized wedges.
Place the chicken breast on one side of the baking sheet and arrange the Brussels sprouts, carrots, and onions on the other side.
Drizzle the olive oil over both the chicken and the vegetables, ensuring everything is lightly coated.
Evenly sprinkle the dried rosemary, thyme, garlic powder, sea salt, and black pepper over the entire tray.
Use your hands or tongs to toss the vegetables in the oil and spices, and rub the seasoning into both sides of the chicken breast.
Roast in the center of the oven for 22 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with charred edges.
Remove from the oven and let the chicken rest for 5 minutes to lock in the juices before slicing it against the grain.
Plate the sliced chicken alongside the roasted vegetables and serve immediately.