Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Trim the ends of the Brussels sprouts and slice them in half.
Cut the chicken breast into 1-inch bite-sized pieces.
In a large bowl, toss the chicken and Brussels sprouts with olive oil, sea salt, black pepper, and garlic powder.
Spread the mixture in a single layer on the prepared baking sheet.
Roast for 20-25 minutes, tossing halfway through, until the chicken is cooked through and the sprouts are caramelized.
While roasting, whisk the balsamic vinegar and honey in a small saucepan over low heat until slightly thickened, about 3-5 minutes.
Remove the pan from the oven, drizzle with the balsamic glaze, and garnish with chopped walnuts.