Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Trim the woody ends off the Brussels sprouts and slice each sprout in half vertically.
Cut the chicken breast into uniform 1-inch cubes to ensure even roasting.
In a large mixing bowl, combine the chicken and Brussels sprouts with the olive oil, sea salt, black pepper, and garlic powder, tossing until thoroughly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the sprouts are cut-side down for maximum caramelization.
Roast for 20 to 25 minutes, tossing halfway through, until the chicken is cooked through and the sprouts are charred and tender.
While the tray roasts, add the balsamic vinegar to a small saucepan over low heat and simmer for about 5 minutes until it reduces into a thick, syrupy glaze.
Remove the sheet pan from the oven, drizzle the warm balsamic glaze over the chicken and sprouts, and garnish with crushed walnuts before serving.