Preheat your oven to 400°F (200°C).
Trim the Brussels sprouts and cut them in half; peel and slice the carrots into rounds, and cut the red onion into small wedges.
On a large baking sheet, toss the Brussels sprouts, carrots, and onions with half of the olive oil, half of the sea salt, half of the black pepper, and all of the dried thyme.
Spread the vegetables in a single layer and roast for 20-25 minutes until they are tender and have developed caramelized edges.
While the vegetables roast, season the chicken breast on both sides with the garlic powder and the remaining sea salt and black pepper.
Heat the remaining olive oil in a stainless steel or cast iron skillet over medium-high heat.
Place the chicken in the hot skillet and sear for 6-7 minutes per side, or until the exterior is golden brown and the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest for 3 minutes before slicing.
Serve the juicy pan-seared chicken alongside the crispy roasted vegetable medley.