Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Trim and halve the Brussels sprouts, slice the carrots into rounds, and cut the red onion into small wedges.
Place the vegetables on the baking sheet and toss with 0.5 tablespoons of avocado oil, half the sea salt, and half the black pepper.
Roast the vegetables in the oven for 20-25 minutes, tossing halfway through, until they are tender and the edges are golden and crispy.
While the vegetables roast, season the chicken breast evenly on both sides with the garlic powder, dried thyme, and the remaining salt and pepper.
Heat the remaining 0.5 tablespoons of avocado oil in a medium 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 internal temperature reaches 165°F (74°C) and the exterior is beautifully browned.
Remove the chicken from the pan and let it rest for 3-5 minutes to lock in the juices.
Plate the chicken alongside the roasted vegetables and drizzle the balsamic vinegar over the entire dish before serving.