Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Trim the Brussels sprouts and cut them in half; peel and slice the carrots into rounds, and cut the red onion into small wedges.
Place the vegetables on the baking sheet and toss with 1 tablespoon of olive oil, 0.25 tsp of sea salt, and the dried thyme.
Roast the vegetables for 20 to 25 minutes, tossing halfway through, until they are tender and the edges are golden brown.
While the vegetables roast, season the chicken breast on both sides with the remaining sea salt, black pepper, and garlic powder.
Heat the remaining 0.5 tablespoon of olive oil in a heavy-bottomed skillet over medium-high heat.
Add the chicken to the pan and sear for 6 to 7 minutes per side, or until the exterior is golden and the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest for 3 minutes before serving alongside the warm roasted vegetables.