Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Trim the ends of the Brussels sprouts and slice them in half lengthwise.
Season the chicken breast with garlic powder, half of the sea salt, and half of the black pepper.
In a bowl, toss the halved Brussels sprouts with olive oil and the remaining salt and pepper until evenly coated.
Place the chicken and Brussels sprouts on the prepared baking sheet, ensuring the sprouts are cut-side down for maximum caramelization.
Roast for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the sprouts are tender and charred.
While roasting, place the walnuts in a dry skillet over medium heat, tossing frequently for 3 to 5 minutes until fragrant and toasted.
Remove the tray from the oven, drizzle the balsamic vinegar over the Brussels sprouts, and toss to coat.
Slice the chicken and serve alongside the glazed sprouts, topped with the crunchy toasted walnuts.