Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Trim and halve the Brussels sprouts, peel and slice the carrots into half-inch rounds, and cut the red onion into thick wedges.
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 caramelized.
While vegetables roast, season the chicken breast evenly on both sides with garlic powder, dried oregano, and the remaining salt and pepper.
Heat the remaining olive oil in a medium skillet over medium-high heat.
Add the chicken to the skillet and sear for 6 to 7 minutes per side, or until the internal temperature reaches 165°F and the exterior is golden brown.
Remove the chicken from the heat and drizzle with fresh lemon juice.
Plate the chicken alongside the roasted vegetables and serve immediately.