Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Pat the chicken breast dry with a paper towel and cut it into 1-inch bite-sized pieces for even cooking.
Prepare the vegetables by cubing the butternut squash into 1/2-inch pieces, halving the Brussels sprouts, and slicing the red onion into wedges.
In a large mixing bowl, toss the chicken and vegetables with the olive oil, sea salt, black pepper, rosemary, thyme, and garlic powder until thoroughly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the pieces aren't overcrowded so they roast rather than steam.
Roast for 20-25 minutes, tossing halfway through, until the chicken is cooked through and the vegetables are tender with golden, caramelized edges.
Remove from the oven and let rest for 2 minutes before serving to allow the juices to redistribute.