Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper.
Trim the ends of the Brussels sprouts and cut them in half, then dice the chicken breast into uniform 1-inch pieces.
Place the chicken and Brussels sprouts on the baking sheet and drizzle with olive oil, then sprinkle with sea salt, black pepper, and garlic powder.
Toss everything together until evenly coated and spread out in a single layer, ensuring the sprouts are cut-side down for maximum caramelization.
Roast in the oven for 20 to 25 minutes, or until the chicken is cooked through and the sprouts are tender and golden brown.
During the final 5 minutes of roasting, scatter the walnuts onto the baking sheet to toast them until fragrant.
Remove from the oven, immediately drizzle the balsamic vinegar over the hot chicken and vegetables, and toss well to glaze before serving.