Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Trim the woody ends off the Brussels sprouts and slice them in half vertically to ensure they get maximum caramelization against the pan.
Season the chicken breast evenly on both sides with the garlic powder and half of the sea salt and black pepper.
In a large mixing bowl, toss the halved Brussels sprouts with the olive oil and the remaining salt and pepper until every piece is lightly coated.
Arrange the chicken and the Brussels sprouts in a single layer on the prepared baking sheet, placing the sprouts cut-side down for the best char.
Roast for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the sprouts are tender with golden-brown edges.
While the tray is in the oven, combine the balsamic vinegar and maple syrup in a small saucepan over medium-low heat.
Simmer the balsamic mixture for about 5 minutes, stirring occasionally, until it reduces into a thick, syrupy glaze that coats the back of a spoon.
Remove the tray from the oven, drizzle the warm balsamic glaze over the sprouts and chicken, and finish with a sprinkle of chopped walnuts for a nutty crunch.