Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Prepare the vegetables by trimming the ends off the Brussels sprouts and halving them, slicing the carrot into 1/2-inch rounds, and breaking the cauliflower into bite-sized florets.
Place the chicken breast and all the prepared vegetables onto the sheet pan in a single layer.
Drizzle the avocado oil over the chicken and vegetables.
Evenly sprinkle the rosemary, thyme, garlic powder, sea salt, and black pepper over everything.
Toss the vegetables with your hands or tongs to ensure they are well-coated, and rub the seasoning into both sides of the chicken.
Roast in the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender and golden-brown.
Remove from the oven and let the chicken rest for 5 minutes before slicing it into strips.
Plate the sliced chicken alongside the roasted vegetables and serve immediately.