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. Cut the chicken breast into 1-inch bite-sized cubes.
In a large bowl, toss the chicken and Brussels sprouts with the olive oil, sea salt, black pepper, and garlic powder until evenly coated.
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 golden brown.
While the chicken and sprouts roast, place the walnuts in a small dry skillet over medium heat. Toast for 3-5 minutes until fragrant, then roughly chop.
In a small bowl, whisk together the balsamic vinegar and raw honey to create a glaze.
Remove the baking sheet from the oven. Drizzle the balsamic glaze over the roasted chicken and sprouts, tossing to coat while still hot.
Transfer to a plate and garnish with the toasted walnuts before serving.