Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Trim and halve the Brussels sprouts, slice the carrots into rounds, and roughly chop the red onion.
Place the vegetables on the baking sheet, drizzle with half of the olive oil, and season with a pinch of salt and pepper.
Roast the vegetables for 20 to 25 minutes, tossing halfway through, until they are tender and slightly caramelized.
While the vegetables roast, season the chicken breast evenly on both sides with garlic powder, dried rosemary, and the remaining salt and pepper.
Heat the remaining olive oil in a cast-iron or stainless steel skillet over medium-high heat.
Place the chicken in the hot skillet and sear for 6 to 8 minutes per side until the exterior is golden brown and the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest for 5 minutes to lock in the juices before slicing.
Plate the sliced chicken alongside the roasted vegetable medley and serve immediately.