Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Trim the Brussels sprouts and halve them, slice the carrots into rounds, and chop the red onion into large chunks.
In a large bowl, toss the vegetables with half of the olive oil, half of the sea salt, pepper, and the dried thyme until evenly coated.
Spread the vegetables in a single layer on the prepared baking sheet and roast for 20-25 minutes until tender and caramelized.
While the vegetables roast, season the chicken breast on both sides with the remaining salt, pepper, and garlic powder.
Heat the remaining olive oil in a cast-iron or stainless steel skillet over medium-high heat.
Place the chicken in the skillet and sear for 5-7 minutes per side, or until the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest for 5 minutes before slicing.
Serve the sliced chicken alongside the roasted vegetables.